HiresTextures: Support parsing DDS files directly

This leaves DDS textures using DXT1/3/5 compressed in-memory, which can
be passed directly to the backend.

Source/Core/VideoCommon/VideoCommon.h

@@ -77,6 +77,15 @@ enum class APIType
   Nothing
 };
 
+// Texture formats that videocommon can upload/use.
+enum class HostTextureFormat : u32
+{
+  RGBA8,
+  DXT1,
+  DXT3,
+  DXT5
+};
+
 inline u32 RGBA8ToRGBA6ToRGBA8(u32 src)
 {
   u32 color = src;