Fix #34461: Inconsistent behavior of "Color Mix Node" and "Alpha Over Node"

Added compatibility option "Straight Alpha Output" to image input node

When this option is enabled, image input node will convert float buffer
to straight alpha.

This is not what you'll usually want with new alpha pipeline, nit this
is needed to preserve compatibility with older files saved in 2.65.
In that version byte image are resulting with straight alpha passing
to the compositor and alpha-overing required extra premultiplication
of inputs.

So, that's why Straight Alpha Output is needed -- it's set in versioning
code for byte node images so they'll still output straight alpha.

This option is currently only available in N-panel.

Additional change: added Alpha Mode for image input node to N-panel.

git-svn-id: https://svn.blender.org/svnroot/bf-blender/trunk/blender@54960 954f8c5b-7b00-dc11-b283-0030488c597c