app_server: attach correct rendering buffer to cloned mask

* When cloning a cached mask, we have to attach the
AGG rendering buffer of the new instance to the AGG clipped
mask object. Before, it was using the buffer description from
the clone source.. which can later disappear at any time.

* Fixes bug #12478

app_server: give Painter direct access to alpha masks

* To use alpha masks in optimized drawing code outside of the AGG
renderer pipeline, we need to allow access to the alpha mask's
underlying buffer:

- AlphaMask gets another method which returns its
clipped_alpha_mask
- clipped_alpha_mask gets a get_hspan() method which returns a span
of alpha values without combining it with anything
- Painter/PainterAggInterface store a pointer to the
clipped_alpha_mask (in addition to the AlphaMask's scanline
container)

app_server: Cleanup of some file locations

app_server: Cleanup of some file locations