lib: Make tile_composite functions take fix15_t C arrays instead of PyObjects

Expose a single function from C++ to Python, tile_composite, that
is parameterized by the blending mode.
Preparation for moving more of tile manipulation code down into C/C++.
lib: Get rid of individual tile_composite_$mode C functions

Just instantiate the specializations directly into function mapping.
Merge branch 'mypaint-head' to sync with MyPaint