fix: remove incorrect Y-flip in postprocess vertex shader

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)

assets/shaders/postprocess.vert.glsl

@@ -6,5 +6,7 @@ void main() {
     // Fullscreen triangle trick: 3 vertices, no vertex buffer
     TexCoord = vec2((gl_VertexIndex << 1) & 2, gl_VertexIndex & 2);
     gl_Position = vec4(TexCoord * 2.0 - 1.0, 0.0, 1.0);
-    TexCoord.y = 1.0 - TexCoord.y; // flip Y for Vulkan
+    // No Y-flip: scene textures use Vulkan convention (v=0 at top),
+    // and NDC y=-1 already maps to framebuffer top, so the triangle
+    // naturally samples the correct row without any inversion.
 }