Initial support for Windows is implemented using the winpty translation
layer. Clipboard support for Windows is provided through the `clipboard`
crate, and font rasterization is provided by RustType.
The tty.rs file has been split into OS-specific files to separate
standard pty handling from the winpty implementation.
Several binary components are fetched via build script on windows
including libclang and winpty. These could be integrated more directly
in the future either by building those dependencies as part of the
Alacritty build process or by leveraging git lfs to store the artifacts.
Fixes#28.
Uses the GL_ARB_blend_func_extended to get single-pass, per-channel
alpha blending. gl_generator is now used instead of gl to enable the
extension.
The background color is removed since that presumably needs to run in a
separate pass.
