[GPU/UI] XeSL readability improvements + float suffix

Use the _xe suffix instead of the xesl_ prefix for quicker visual
recognition of identifiers, also switch to snake_case for consistency.

Also add the f suffix to float32 literals because the Metal Shading
Language is based on C++.

src/xenia/gpu/shaders/fxaa_extreme.cs.hlsl

@@ -1,5 +1,5 @@
 #define FXAA_QUALITY__PRESET 39
-#define FXAA_QUALITY__SUBPIX 1.0
-#define FXAA_QUALITY__EDGE_THRESHOLD 0.063
-#define FXAA_QUALITY__EDGE_THRESHOLD_MIN 0.0312
+#define FXAA_QUALITY__SUBPIX 1.0f
+#define FXAA_QUALITY__EDGE_THRESHOLD 0.063f
+#define FXAA_QUALITY__EDGE_THRESHOLD_MIN 0.0312f
 #include "fxaa.hlsli"