Render underline and strikeout rects in batches

Currently Alacritty requires a separate `draw` call to OpenGL whenever a
new rectangle is rendered to the screen. With many rectangles visible,
this has a significant impact on rendering performance.

Instead of using separate draw calls, the new `RectRenderer` will build
a batch of rectangles for rendering. This makes sure that multiple
rectangles can be grouped together for single draw calls allowing a
reduced impact on rendering time.

Since this change is OpenGL 2 friendly, it should not make it more
complicated to transition away from the 3.3+ requirements like an
alternative instancing based implementation might have.

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