shader_recompiler: Improve shader exports accuracy (part 1) (#3447)

* video_core: support for RT layer outputs

- support for RT layer outputs
- refactor for handling of export attributes
- move output->attribute mapping to a separate header

* export: Rework render target exports

- Centralize all code related to MRT exports into a single function to make it easier to follow
- Apply swizzle to output RGBA colors instead of the render target channel.
  This fixes swizzles on formats with < 4 channels

For example with render target format R8_UNORM and COMP_SWAP ALT_REV the previous code would output

frag_color.a = color.r;

instead of

frag_color.r = color.a;

which would result in incorrect output in some cases

* vk_pipeline_cache: Apply swizzle to write masks

---------

Co-authored-by: polyproxy <47796739+polybiusproxy@users.noreply.github.com>

src/video_core/amdgpu/pixel_format.h

@@ -104,13 +104,18 @@ enum class NumberConversion : u32 {
     Uint32ToUnorm = 6,
 };
 
-struct CompMapping {
-    CompSwizzle r;
-    CompSwizzle g;
-    CompSwizzle b;
-    CompSwizzle a;
+union CompMapping {
+    struct {
+        CompSwizzle r;
+        CompSwizzle g;
+        CompSwizzle b;
+        CompSwizzle a;
+    };
+    std::array<CompSwizzle, 4> array;
 
-    auto operator<=>(const CompMapping& other) const = default;
+    bool operator==(const CompMapping& other) const {
+        return array == other.array;
+    }
 
     template <typename T>
     [[nodiscard]] std::array<T, 4> Apply(const std::array<T, 4>& data) const {