Added RGBA|RGB channels toggle to sequencer preview

Main purpose of this is to be more compatible with older
versions of blender (before alpha cleanup) where sequencer
used to display premultiplied image on an straight opengl
viewport.

Now sequencer preview would behave closer to image editor
However adding Alpha and R|G|B displays is not so simple
because sequencer is using 2D textures. Would be nice to
implement this options as well, but this is not so much
important IMO.

This hall fix
- #34453: VSE: Subtract function does not work properly

TODO: Make RGBA display default for our startup.blend

git-svn-id: https://svn.blender.org/svnroot/bf-blender/trunk/blender@54904 954f8c5b-7b00-dc11-b283-0030488c597c