fix: remove Y-flip counter-hacks in FSR shaders; invert mouse by default; FSR1 disables MSAA

FSR EASU and FSR2 sharpen fragment shaders had a manual Y-flip to undo
the now-removed postprocess.vert flip.  Strip those since the vertex
shader no longer flips, making all postprocess paths consistent.

Also flip the default mouse Y-axis to match user expectation (mouse
down = look up / flight-sim style) and make FSR1 disable MSAA on
enable, matching FSR2 behaviour (FSR provides its own spatial AA).

assets/shaders/fsr2_sharpen.frag.glsl

@@ -10,9 +10,7 @@ layout(push_constant) uniform PushConstants {
 } pc;
 
 void main() {
-    // Undo the vertex shader Y flip (postprocess.vert flips for Vulkan overlay,
-    // but we need standard UV coords for texture sampling)
-    vec2 tc = vec2(TexCoord.x, 1.0 - TexCoord.y);
+    vec2 tc = TexCoord;
 
     vec2 texelSize = pc.params.xy;
     float sharpness = pc.params.z;

assets/shaders/fsr_easu.frag.glsl

@@ -21,9 +21,7 @@ vec3 fsrFetch(vec2 p, vec2 off) {
 }
 
 void main() {
-    // Undo the vertex shader Y flip (postprocess.vert flips for Vulkan overlay,
-    // but we need standard UV coords for texture sampling)
-    vec2 tc = vec2(TexCoord.x, 1.0 - TexCoord.y);
+    vec2 tc = TexCoord;
 
     // Map output pixel to input space
     vec2 pp = tc * fsr.con2.xy; // output pixel position