Make a public CoglBitmapError enum

There are many places in the texture backend that need to do
conversion using the CoglBitmap code. Currently none of these
functions can throw an error but they do return a value to indicate
failure. In future it would make sense if new texture functions could
throw an error and in that case they would want to use a CoglBitmap
error if the failure was due to the conversion. This moves the
internal CoglBitmap error from the quartz backend to be public in
cogl-bitmap.h so that it can be used in this way.

cogl/cogl-bitmap-pixbuf.c

@@ -77,22 +77,6 @@ _cogl_bitmap_premult (CoglBitmap *dst_bmp)
 
 #ifdef USE_QUARTZ
 
-/* lacking GdkPixbuf and other useful GError domains, define one of our own */
-
-#define COGL_BITMAP_ERROR cogl_bitmap_error_quark ()
-
-typedef enum {
-  COGL_BITMAP_ERROR_FAILED,
-  COGL_BITMAP_ERROR_UNKNOWN_TYPE,
-  COGL_BITMAP_ERROR_CORRUPT_IMAGE
-} CoglBitmapError;
-
-GQuark
-cogl_bitmap_error_quark (void)
-{
-  return g_quark_from_static_string ("cogl-bitmap-error-quark");
-}
-
 gboolean
 _cogl_bitmap_get_size_from_file (const char *filename,
                                  int        *width,