Added support for DXT1/3/5 and BC5/7 compressed textures via love.image.CompressedData. Some internals are still iffy.

- added a new CompressedData type to love.image
- added love.image.isCompressed(file or data)
- love.graphics.newImage automatically tests for compression when loading a file
- added love.graphics.isSupported strings for DXT, BC5, and BC7

Compressed textures meant to be used on the GPU offer huge performance gains when
- loading the texture from a file
- loading the texture from RAM to VRAM
- loading mipmaps
- rendering

--HG--
branch : image-CompressedData

src/common/runtime.cpp

@@ -469,6 +469,7 @@ StringMap<Type, TYPE_MAX_ENUM>::Entry typeEntries[] =
 
 	// Image
 	{"ImageData", IMAGE_IMAGE_DATA_ID},
+	{"CompressedData", IMAGE_COMPRESSED_DATA_ID},
 
 	// Audio
 	{"Source", AUDIO_SOURCE_ID},