1784385 - WR performance - Remove unused GLSL code for get_image_quad_uv and associated perspective interpolation

Status: ASSIGNED

(Core :: Graphics: WebRender, defect, P3)

Description

[Ashley Hale [:ahale]]

WebRender used to use inverse transforms in GLSL shaders that sampled from render targets, this code is no longer used but is a performance burden and maintenance burden, it makes the GLSL shader code non-trivially more complex without a benefit since it is unused now.

This also causes a surprising and undocumented breakage in reftests when using static images to test shaders meant to be used with a render target, because static images do not fill in this transform information in GpuCache, resulting in broken rendering if this code is not commented out (ran into this when working on bug 1671784).

Since this code is not used anymore, it should be removed to clean up the codebase.

It's not known how much performance benefit would come from removing this code in the shaders, but probably something (it incurs a per vertex cost and per pixel cost).