decodes a PVR image
The decode_header and _decode
has these elements:
"image":object - image object \- not decode_header
"alpha":object - decoded alpha /
"type":"image/x-pvr" - image type
"xsize":int - horisontal size in pixels
"ysize":int - vertical size in pixels
"attr":int - texture attributes
"global_index":int - global index (if present)