The new paint cursor (introduced in rBe0c792135adf) mixed 3d and 2d
drawing leading to asserts [e.g. when tablet pressure sensitivity was
enabled for size, see D5820 also].
We could get away with always drawing in 3D [using vertformat with
comp_len 3 / GPU_SHADER_3D_UNIFORM_COLOR / imm_draw_circle_wire_3d],
even if in the Image Editor, but this patch clearly separates what is
drawn in 3d and what is in 2d.
part of T69957
Reviewers: jbakker
Differential Revision: https://developer.blender.org/D5836
078fcc6253