gpu: fix zero-source alpha blending on swizzled targets

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

src/graphics/host_gpu/renderer/pipeline/blendMapping.h

@@ -16,6 +16,7 @@ enum class BlendMappingSupport {
 	Direct,
 	SourceAlpha, // Requires logical alpha in the second blend source.
 	SourceAlphaOne, // Source alpha for RGB, one for alpha, with a shared destination equation.
+	SourceAlphaZero, // Source alpha for RGB, zero for alpha, with a shared destination equation.
 	Unsupported,
 };