MT support in core filters, part 2

This pull request adds MT support in most core filters.All modified filters are declared as re-entrant and use new allocation API to decrease memory usage. Some filters had also been slightly refactored, Visual C++ types replaced with stdint.h types. Nothing too serious though.

What's **not** done:
* Audio filters - no idea how audio works.
* Animate - should probably be marked as serialized (default) or rewritten completely. Right now it caches created clips which isn't really mt-friendly.
* Conditional filters - they set global variables like current_frame, which won't work with MT well.
* Splice - it's a NonCached (NICE) filter as far as video goes, but not sure about audio (probably safe).
* Dissolve - NICE video, not sure about audio (probably safe).
* Text-overlay functions (e.g. subtitle) - not really sure how it works, will be replaced at some point anyway.
* ColorYUV - will be rewritten completely.
* Resample/overlay - done by innocenat in other PR.