also known as: broken multicam strip caused by other fix.
Calculated render_size where it belongs (within the glow effect) and
restored old functionality.
also: renamed render_size to preview_render_size at all relevant places,
where the naming wasn't used correctly.
Hopefully it's now a little bit more clear.
render_size := render size from scene (just rescales width/height)
preview_render_size := preview render size from sequencer preview,
controls the resolution and the use of sequencer proxy sources
f890b00851