Blender Git Loki

Cleanup: Add functions to check the first brush step and symmetry passes

This adds three functions to check the state of the stroke in the
StrokeCache, removing the references to first_time and
mirror_symmetry_pass from the code. This makes easier to understand what
each code path is doing inside of each tool.

Some tools were using mirror_symmetry_pass incorrectly, so this should
also fix unreported bugs with radial and tiling symmetry related to that.

Reviewed By: sergey

Differential Revision: https://developer.blender.org/D8164