Add support for uploading JP2 files.

Recognizes .jp2, .j2c and .j2k extensions.
Adds image/jp2 files to file picker image filter (windows and Mac,
windows apparently already showed them).
Show preview for jpeg 2000 files.
Fixes error reporting for failed image uploads.
Enforces a power-of-two size for jpeg 2000 files (seemed to make sense to do that).

indra/llimage/llimage.cpp

@@ -1239,6 +1239,7 @@ file_extensions[] =
 {
 	{ "bmp", IMG_CODEC_BMP },
 	{ "tga", IMG_CODEC_TGA },
+	{ "j2k", IMG_CODEC_J2C },
 	{ "j2c", IMG_CODEC_J2C },
 	{ "jp2", IMG_CODEC_J2C },
 	{ "texture", IMG_CODEC_J2C },