Allow use of multiple fonts acting together like a fallback stack,
where if a glyph is not found in one it can be retrieved from another.
See D12622 for much more detail
Differential Revision: https://developer.blender.org/D12622
Reviewed by Brecht Van Lommel
Internally many offsets for BLF were integers but exposed as floats,
since these are used in pixel-space, many callers were converging them
back to integers. Simplify logic by using ints.
Moves all `interface_region*` files to C++ except for the tooptip region
which is slightly more complicated. Also move a few other files as well.
This helps to simplify and speed up code, especially through the use
of better C++ data structures. This change builds on all platforms on
the buildbot.
