Draft: render/gles2: unify fragment shader

Instead of having 3 different fragment shaders for textures and 1 more for quads, unify them all and compile different variants via a SOURCE constant.

In the future, we will have more variant dimensions: pre-multiplied alpha on/off, color transformation, etc. It will become inpractical to have one file per combination.

Weston has a similar approach: https://gitlab.freedesktop.org/wayland/weston/-/blob/main/libweston/renderer-gl/fragment.glsl

Depends on !3803 (merged) and !3807 (merged)