Fix trim functionality on Windows, clean up GL entrypoint querying
* code to query GL proc addresses now lives in vogl_gl_replayer. This probably isn't the ideal location for it, but this a common class among the two different locations that implemented a load_gl() function and the replayer is the class which most closely needs these entrypoints for taking snapshots, so this seems like a reasonable place to put the code.

* The trim functionality is a difficult one to identify how to query the GL proc addresses correctly, as we don't necessarily know where the call to wglMakeCurrent(..) will occur (could be in creating a snapshot or as part of the calls in the replayed frames). My solution is to requery the proc addresses at the start of writing the trim file. This seemed the easiest and most logical place to do it.