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.

2 files changed, 5 insertions, 1 deletions

diff --git a/alacritty/res/rect.f.glsl b/alacritty/res/rect.f.glsl
index edea07dc..945eaf2d 100644
--- a/alacritty/res/rect.f.glsl
+++ b/alacritty/res/rect.f.glsl
@@ -1,6 +1,6 @@
 #version 330 core
 
-uniform vec4 color;
+flat in vec4 color;
 
 out vec4 FragColor;
 
diff --git a/alacritty/res/rect.v.glsl b/alacritty/res/rect.v.glsl
index 02be0bee..bf9a97d3 100644
--- a/alacritty/res/rect.v.glsl
+++ b/alacritty/res/rect.v.glsl
@@ -1,7 +1,11 @@
 #version 330 core
 layout (location = 0) in vec2 aPos;
+layout (location = 1) in vec4 aColor;
+
+flat out vec4 color;
 
 void main()
 {
+    color = aColor;
     gl_Position = vec4(aPos.x, aPos.y, 0.0, 1.0);
 }