Fix selection starting in first cell

When selecting to the top and starting in the first cell, alacritty
would crash. These cases have been fixed and now selection should be
completely working.

out

@@ -0,0 +1,104 @@
+   Compiling alacritty v0.1.0 (file:///home/undeadleech/programming/rust/alacritty)
+warning: unused variable: `count`
+  --> src/input.rs:69:26
+   |
+69 |     fn scroll(&mut self, count: isize) {}
+   |                          ^^^^^ help: consider using `_count` instead
+   |
+   = note: #[warn(unused_variables)] on by default
+
+warning: unused variable: `previous_scroll_limit`
+   --> src/grid/mod.rs:217:13
+    |
+217 |         let previous_scroll_limit = self.scroll_limit;
+    |             ^^^^^^^^^^^^^^^^^^^^^ help: consider using `_previous_scroll_limit` instead
+
+warning: value assigned to `start` is never read
+   --> src/selection.rs:184:13
+    |
+184 |         let mut start = initial_expansion.start;
+    |             ^^^^^^^^^
+    |
+    = note: #[warn(unused_assignments)] on by default
+
+warning: value assigned to `end` is never read
+   --> src/selection.rs:185:13
+    |
+185 |         let mut end = initial_expansion.end;
+    |             ^^^^^^^
+
+warning: method is never used: `pop`
+  --> src/grid/storage.rs:50:5
+   |
+50 |     pub fn pop(&mut self) -> Option<T> {
+   |     ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
+   |
+   = note: #[warn(dead_code)] on by default
+
+warning: method is never used: `swap`
+  --> src/grid/storage.rs:69:5
+   |
+69 |     pub fn swap(&mut self, a: usize, b: usize) {
+   |     ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
+
+    Finished dev [unoptimized + debuginfo] target(s) in 9.82 secs
+     Running `target/debug/alacritty`
+thread 'main' panicked at 'attempt to subtract with overflow', src/index.rs:414:17
+stack backtrace:
+   0: std::sys::unix::backtrace::tracing::imp::unwind_backtrace
+             at libstd/sys/unix/backtrace/tracing/gcc_s.rs:49
+   1: std::sys_common::backtrace::_print
+             at libstd/sys_common/backtrace.rs:71
+   2: std::panicking::default_hook::{{closure}}
+             at libstd/sys_common/backtrace.rs:59
+             at libstd/panicking.rs:207
+   3: std::panicking::default_hook
+             at libstd/panicking.rs:223
+   4: std::panicking::rust_panic_with_hook
+             at libstd/panicking.rs:402
+   5: std::panicking::begin_panic_fmt
+             at libstd/panicking.rs:349
+   6: rust_begin_unwind
+             at libstd/panicking.rs:325
+   7: core::panicking::panic_fmt
+             at libcore/panicking.rs:72
+   8: core::panicking::panic
+             at libcore/panicking.rs:51
+   9: <alacritty::index::Column as core::ops::arith::SubAssign<usize>>::sub_assign
+             at src/index.rs:414
+  10: alacritty::selection::Selection::span_simple
+             at src/selection.rs:283
+  11: alacritty::selection::Selection::to_span
+             at src/selection.rs:167
+  12: alacritty::term::Term::renderable_cells::{{closure}}
+             at src/term/mod.rs:1058
+  13: <core::option::Option<T>>::and_then
+             at /checkout/src/libcore/option.rs:612
+  14: alacritty::term::Term::renderable_cells
+             at src/term/mod.rs:1057
+  15: alacritty::display::Display::draw::{{closure}}
+             at src/display.rs:368
+  16: alacritty::renderer::QuadRenderer::with_api
+             at src/renderer/mod.rs:658
+  17: alacritty::display::Display::draw
+             at src/display.rs:360
+  18: alacritty::run
+             at src/main.rs:203
+  19: alacritty::main
+             at src/main.rs:62
+  20: std::rt::lang_start::{{closure}}
+             at /checkout/src/libstd/rt.rs:74
+  21: std::panicking::try::do_call
+             at libstd/rt.rs:59
+             at libstd/panicking.rs:306
+  22: __rust_maybe_catch_panic
+             at libpanic_unwind/lib.rs:102
+  23: std::rt::lang_start_internal
+             at libstd/panicking.rs:285
+             at libstd/panic.rs:361
+             at libstd/rt.rs:58
+  24: std::rt::lang_start
+             at /checkout/src/libstd/rt.rs:74
+  25: main
+  26: __libc_start_main
+  27: _start

src/selection.rs

@@ -279,8 +279,31 @@ impl Selection {
                 });
             }
         } else if start.line < end.line {
-            start.col -= 1;
             end.col += 1;
+            if start.col > Column(0) {
+                start.col -= 1;
+            }
+            // Special case for when a selection is started
+            // in the first cell of a line
+            else {
+                let ty = match end_side {
+                    Side::Left => SpanType::ExcludeTail,
+                    Side::Right => SpanType::Inclusive,
+                };
+
+                // Switch start cell to last cell of previous line
+                if start_side == Side::Left {
+                    start.line += 1;
+                    start.col = cols - 1;
+                }
+
+                return Some(Span {
+                    ty,
+                    cols,
+                    front: start,
+                    tail: end,
+                });
+            }
         }
 
         let (front, tail, front_side, tail_side) = if start > end {