Add tablet data to mouse input.

[WIP] Continuous grab for tablets

This revision modifies mouse and tablet handling to allow both to interchangably accumulate during continuous grab. For tablet input in pen mode, grab accumulates over the change in position while the tablet is in range. For a tablet in mouse mode, grab accumulates as a normal mouse accumulates.

Also adds ability for operators to force continuous grab. This is necessary for the walk operator which has been rewritten to take advantage of tablet continuous grab.

Differential Revision: https://developer.blender.org/D12408

Cleanup

Converted walk operator to use continuous grab. Mouse and tablet are now opaque to the operator handling.

Continous grab has to be forced even if user has disabled it.

Continuous grab mostly working for Wintab and Windows Ink. Some minor race conditions for Windows Ink when a pen quickly enters then leaves range shortly after a mouse event.

# Conflicts:
# intern/ghost/intern/GHOST_SystemWin32.cpp

Cleanup: remove unmodified return variable.

Supply tablet data with

Consolidate repeat actions.

Improve cursor wrap reliability.

Cleanup: replace defines with static const in walk/fly operators.

No functional changes.

Fix Windows cursor wrap jumps.