[client] egl: refactor damage mesh generation into desktop_rects

This mesh will later be used to render only damaged portions of the desktop.
We also moved the coordinate transformation for damage overlay into a matrix
and computed by the shader.

client/renderers/EGL/CMakeLists.txt

@@ -43,6 +43,7 @@ add_library(renderer_EGL STATIC
 	texture_dmabuf.c
 	model.c
 	desktop.c
+	desktop_rects.c
 	cursor.c
 	draw.c
 	splash.c