Verify sampling of framebuffers in post-processing stages

[jjhembd]

Many of our post-processing stages are sampling the rendered framebuffers with a texture coordinate which is off by half a pixel.

Our current post-processing shaders sample the rendered depth and color framebuffers via the v_textureCoordinates varying. This varying takes values in [0, 1] across the full-screen quad.

At each fragment, the value of v_textureCoordinates equal the coordinate of the pixel's boundary, rather than its center. This leads to the following errors:

• Depth values do not correspond to the computed X and Y coordinates. The depth buffer (in WebGL2) only supports nearest sampling, meaning that the value sampled from the texture is the depth at the pixel center, whereas X and Y are at the pixel boundary.
• Interpolation errors in v_textureCoordinates may lead to unpredictability: the pixel just across the boundary may be selected instead of the expected one.

A better option may be to use gl_FragCoord.xy rather than a varying. gl_FragCoord consistently describes the position of the center of the current fragment. See #12201 for an example of this approach, as well as the impact on the post-processing result.

Some things we need to verify:

• Revisit some of the depth buffer gradient QCs from Ambient Occlusion doesn't work！ #10106 (comment) to see if they are fixed by sampling pixel centers.
• Confirm whether other post-processing stages are affected by the coordinate error.