Similar to the latest commits for GimpBrush:
- gimp_pattern_get_buffer() returns a GeglBuffer and allow getting a scaled
version of the pattern.
- Old gimp_pattern_get_pixels() is made private.
- Moved GimpPattern into its own file and store the buffer to avoid re-querying
it through PDB continuously.
No as for the widget to select a pattern:
- Preview frame ensured to be square.
- Default size increased.
- Drawing code using the new gimp_pattern_get_buffer().
- Cleaned up code.
