Zooming in `view_zoomstep_apply_ex` ignores any kind of v2d limits,
which are only applied later in `UI_view2d_curRect_validate`. After
zooming in all the way, further attempted zooms get undone by
`ui_view2d_curRect_validate_resize` in the following manner:
1. VSE has no `V2D_LIMITZOOM` flag set, so a v2d that is too small tries
to grow to `v2d->min[0]`.
2. Since `V2D_KEEPOFS_X` flag is set so that resizing the VSE can be
anchored to the left, the way that the v2d is altered to grow to the
minimum width (10 frames) is by increasing only `cur->xmax` instead
of both sides.
Fix by gating keepofs logic behind a similar `do_keepofs` to the one
which was added to `view_zoomstep_apply_ex` in 385a8a4d6a.
This is quite safe and regression-proof since every space but the
VSE has `V2D_ZOOM_IGNORE_KEEPOFS` unset, in which case the logic is
unchanged.
Ref: 7d9499bdc4, 8b36cf3eac, 385a8a4d6a, 28b1a33e16.
a7f6d80395