A method `render_rect` has been added that allows drawing a rectangle
over anything in the terminal. This is used by the visual bell to draw a
big rectangle over everything.
To achieve this a new shader has been added. Shaders are now kept around
until the program is dropped. Then whenever a rectangle draw call is
issued, the shader is swapped from the default shader to the rectangle
drawing shader.
After the shader has been swapped and the rectangle has been drawn,
uniforms (like projection) and blending function are reset to make sure
the rendering can be continued on the default shader.
All current instances of visual_bell in the rendering have been removed,
it is now all done exclusively through the rectangle rendering function.
Currently the shaders are swapped every time `render_rect` is called, so
when rendering multiple rectangles (potentially for underline /
strikethrough) there would be a loads of shader swaps that could be
potentially very expensive.
So before using `render_rect` for a different application, the shader
swap should be removed from the `render_rect` method.
As long as the visual bell's alpha value is 0, this has no effect on
performance because no rectangle is rendered.
