Blender Git Loki

VSE: Fix proxy images have incorrect size

This bug happens when media size doesn't match scene size and proxies
are enabled. This is because some strips are rendered in preview size
already - effects, mask or strips using proxies.
If strip input is prescaled it needs to be known when transforming
images, which does also scaling to preview size as one operation.

whether image needs prescaling is defined in function
`seq_need_scale_to_render_size()`

Problem was introduced in 571362642201. Previously all images were
scaled to fit into preview, and this was considered as baseline for
further image transformation.

Reviewed By: sergey

Differential Revision: https://developer.blender.org/D10183