Blender Git Loki

VSE: Fix handle size calculation

Handle width calculation was incorrect in drawing code. This caused
handles to be invisible when zoomed out.

After fixing math, handles become too large, so now they are constrained
to quarter of strip width, which feels more natural and represents
clickable area more closely.

`sequence_handle_size_get_clamped()` did not return size in pixels, but
in 2D-View space, this comment was corrected.