postprocess.vert.glsl had `TexCoord.y = 1.0 - TexCoord.y` which
inverted the vertical sampling of the scene texture. Vulkan textures
use v=0 at the top, matching framebuffer row 0, so no flip is needed.
The camera already flips the projection matrix (mat[1][1] *= -1) so
the scene is rendered correctly oriented; the extra inversion in the
postprocess pass flipped FXAA output upside down.
Fixes: FXAA shows camera upside down
Also fixes: FSR1 upscale and FSR3 sharpen passes (same vertex shader)
