Vulkan: Set alpha channel of swap chain buffers to 1.0.

Copying the alpha channel from the game causes issues with frame dumping,
since we're using a buffer directly from the GPU as a source for AVIDump.

Source/Core/VideoBackends/Vulkan/Renderer.cpp

@@ -1521,7 +1521,7 @@ bool Renderer::CompileShaders()
 
     void main()
     {
-      ocol0 = texture(samp0, uv0);
+      ocol0 = float4(texture(samp0, uv0).xyz, 1.0);
     }
   )";