Blender Git Loki

Fix for Grease Pencil and negative frames

While investigating T44412, I noticed some weirdness going on when trying to
draw on frame 0 (i.e. strokes were getting added to frame 1 instead). Clearly,
this seemed like an off-by-one error related to clamping to prevent negative
frames which was also excluding frame 0.

This commit reverts the fixes made for T36831 in:
rBf18f2fbb33d90ecc91e6f3d063cb9f97f217e808

After thinking this over, I think these checks against drawing on negative
frames aren't needed. Even if the current userpref setting doesn't allow
navigating to negative frames, this may not be true for other users that
may work on the same file (in a team environment). Also, negative frame
values can get set via the dopesheet.