Fix a bug where intermediate phase outputs could get too low height.

Basically our aspect ratio adjustment and lack of proper roundoff
could conspire to let e.g. mix(1280x720, 939x939) = 1669x938,
which is obviously wrong. I could probably have fixed it with
an extra lrintf(), but it seems more robust to simply avoid the
extra conversion (ie., never convert height -> width -> height).

Add a unit test to verify.
Fix a Clang warning.

Patch by Dan Dennedy.
If all inputs to an effect have the same input size, use that instead of fitting to the aspect.

The common case here is one-input effects on overlays.
Reported by Christophe Thommeret.
Give SizeStoringEffect an effect_type_id(), for easier debugging of the unit test.
In resizing effects, add the notion of a “virtual output size”.

This is the size that the effect wants to be perceived as for the next
effect in the chain, which is useful if you e.g. have a blur and then
padding. Even though the blur might rescale the image down from e.g.
512x512 to 128x128 before blurring (in case of a large blur), and this
size is what actually comes out in the texture at the other end,
it still wants to be treated as a 512x512 image when adding padding.

Reported by Christophe Thommeret.
Give the alpha enums somewhat better/more consistent names, and shuffle them around a bit.
Rename the OutputAlphaFormat enums; they had gotten inconsistent after the last alpha-related rename.
Add a new alpha handling method, INPUT_PREMULTIPLIED_ALPHA_KEEP_BLANK.

This should fix most of the problems where you only process inputs
without alpha, but the framework still inserts an AlphaDivisionEffect
at the end because it loses track of the blank alpha state throughout
the pipeline and the output expects postmultiplied alpha.

There's one important case left, though: MixEffect will always reset
tha alpha state to premultiplied. We should probably fix that too
at some later stage.
Merge branch 'master' of http://git.sesse.net/movit into opengl
Adjust to Movit OutputAlphaFormat enum change.