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.h

@@ -94,6 +94,38 @@ cogl_bitmap_get_size_from_file (const char *filename,
 gboolean
 cogl_is_bitmap (CoglHandle handle);
 
+/**
+ * COGL_BITMAP_ERROR:
+ *
+ * #GError domain for bitmap errors.
+ *
+ * Since: 1.4
+ */
+#define COGL_BITMAP_ERROR (cogl_bitmap_error_quark ())
+
+/**
+ * CoglBitmapError:
+ * @COGL_BITMAP_ERROR_FAILED: Generic failure code, something went
+ *   wrong.
+ * @COGL_ERROR_UNKNOWN_TYPE: Unknown image type.
+ * @COGL_ERROR_CORRUPT_IMAGE: An image file was broken somehow.
+ *
+ * Error codes that can be thrown when performing bitmap
+ * operations. Note that gdk_pixbuf_new_from_file() can also throw
+ * errors directly from the underlying image loading library. For
+ * example, if GdkPixbuf is used then errors #GdkPixbufError<!-- -->s
+ * will be used directly.
+ *
+ * Since: 1.4
+ */
+typedef enum {
+  COGL_BITMAP_ERROR_FAILED,
+  COGL_BITMAP_ERROR_UNKNOWN_TYPE,
+  COGL_BITMAP_ERROR_CORRUPT_IMAGE
+} CoglBitmapError;
+
+GQuark cogl_bitmap_error_quark (void);
+
 G_END_DECLS
 
 #endif /* __COGL_BITMAP_H__ */