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);
     }
   )";
 

Source/Core/VideoBackends/Vulkan/Renderer.h

@@ -118,6 +118,10 @@ private:
 
   // Shaders used for clear/blit.
   VkShaderModule m_clear_fragment_shader = VK_NULL_HANDLE;
+
+  // NOTE: The blit shader here is used for the final copy from the source buffer(s) to the swap
+  // chain buffer for presentation. It ignores the alpha channel of the input image and sets the
+  // alpha channel to 1.0 to avoid issues with frame dumping and screenshots.
   VkShaderModule m_blit_fragment_shader = VK_NULL_HANDLE;
 
   // Texture used for screenshot/frame dumping