sl2: Version the screen language cache.

This ensures that when we prepare screens - such as after a language
change - the cache is dropped. If we don't do this, sl2 can use old
values for displayable arguments and const displayables.
sl2: Fix analysis of Python if statements.
sl2: Fix FieldValue equality.
Remove constant offsets from video timestamps.

We had previously been assuming the audio started at 0, while
respecting the video PTS. If the PTS of the first video frame was
non-zero, this would lead to the video being delayed behind the audio
and never catching up.

Fixes #386.
sl2: Fix fixed.

The SL fixed statement was calling renpy.display.layout.Fixed with
the layout parameter. Since Fixed also supplied a layout parameter
we'd get a crash.
Distribute the project given to android_build.

This fixes a bug where we would distribute the current project
instead of the one supplied on the command line, causing things
not to work.
Add the selected parameter to the common screen.
Revert parameterization of launcher screens.

We want to be able to pass in arbitrary arguments, so we can sub
them into text.
Ignore .rpyc files that cannot be loaded.

This makes it possible to version down if the .rpy file exists and
do not use any newer features.
Update changelog.

Conflicts:
	sphinx/source/changelog.rst