Collection of theoretical improvements for ATI cards, as well as a couple memory leaks. Thanks to Shyotl for bringing them up and offering solutions.

Signed-off-by: Beeks <HgDelirium@gmail.com>

indra/llimage/llimagepng.cpp

@@ -135,6 +135,7 @@ BOOL LLImagePNG::encode(const LLImageRaw* raw_image, F32 encode_time)
 	if (! pngWrapper.writePng(raw_image, mTmpWriteBuffer))
 	{
 		setLastError(pngWrapper.getErrorMessage());
+		delete[] mTmpWriteBuffer; 
 		return FALSE;
 	}