Variables renaned:
- cfra -> timeline_frame
- nr -> frame index
- cfra_over -> overlap_frame
Function seq_give_stripelem_index was renamed to seq_give_frame_index.
Replace BKE_sequencer wirh SEQ_render or SEQ_proxy prefixes.
In cases where function is very generic, only SEQ prefix is used.
Reviewed By: sergey
Differential Revision: https://developer.blender.org/D9439
Move functions closely related to rendering images and proxies into
render.c and proxy.c files. render.h and proxy.h are created for
functions used internally.
There should be no functional changes.
