Enable KTX sRGB formats

- When parsing, set m_srgb flag on the ImageContainer if the image
  is in an sRGB format.

- When writing, take a new srgb parameter. If set, use the appropriate
  sRGB format.

include/bimg/bimg.h

@@ -541,6 +541,7 @@ namespace bimg
 		, uint32_t _depth
 		, uint8_t _numMips
 		, uint32_t _numLayers
+		, bool _srgb
 		, const void* _src
 		, bx::Error* _err = NULL
 		);