gpu: fix zero-source alpha blending on swizzled targets

Co-authored-by: Çağatay Güçlü <33038942+defektu@users.noreply.github.com>

src/graphics/shader/shader.h

@@ -158,7 +158,7 @@ struct ShaderComputeInputInfo: ShaderWorkgroupInputInfo {
 	ShaderStageRuntime stage;
 };
 
-enum class ShaderAlphaBlendSource : uint8_t { None, SourceAlpha, SourceAlphaOne };
+enum class ShaderAlphaBlendSource : uint8_t { None, SourceAlpha, SourceAlphaOne, SourceAlphaZero };
 
 struct ShaderPixelInputInfo {
 	uint32_t                                       interpolator_settings[32]    = {0};