cogl_read_pixels: Fix the format used in GLES2

When converting the data in cogl_read_pixels it was using bmp_format
instead of the format passed in to the function. bmp_format is the
same as the passed in format except that it always has the premult bit
set. Therefore the conversion would not handle premultiply correctly.

http://bugzilla.clutter-project.org/show_bug.cgi?id=2414

cogl/cogl.c

@@ -655,7 +655,7 @@ _cogl_read_pixels_with_rowstride (int x,
          allocating its own buffer so we have to copy the data
          again */
       if ((dst_bmp = _cogl_bitmap_convert_format_and_premult (tmp_bmp,
-                                                              bmp_format)))
+                                                              format)))
         {
           _cogl_bitmap_copy_subregion (dst_bmp,
                                        bmp,