Refactor code to use `eSpaceSeq_Proxy_RenderSize` or corresponding `IMB_Proxy_Size`
enum items directly.
`SEQ_PROXY_RENDER_SIZE_100` has assigned value 99 to distinguish from `SEQ_PROXY_RENDER_SIZE_FULL`.
This caused error in image size calculation and because of that image had to be scaled.
Author: EitanSomething
Reviewed By: ISS
Differential Revision: http://developer.blender.org/D6368