When holding CTRL using the draw tool to erase, the size of the cursor
was using the size of the eraser brush. This is not the expected behavior
when using the eraser from the draw tool. It should respect the size
of the brush used by the draw tool instead.
This fixes the issue by computing the right size when the eraser operation
is invoked. The size is then stored in a runtime field, so that the cursor
rendering callback can use the cached size.
Pull Request: https://projects.blender.org/blender/blender/pulls/125225
d0ff8d89c9