gl: move gl(base)mixer to library

commit fd7debc5

glmixer: don't add rgba templates by default

Subclasses may want to override the pad template with different formats
or with a different pad subclass.

The original beahviour is still available by calling
gst_gl_mixer_class_add_rgba_pad_templates() in _class_init() of the
subclass.

commit f61104cd

glmixer: remove set_caps() vfunc

All of its implementors can be moved to gl_start/stop() from
GstGLBaseMixer instead.

commit 21f2e3e5

glmixer: remove reset vfunc

This functionality is covered by GstGLBaseMixer's gl_stop() vfunc.

commit 1ea37e4e

gl/mixer: make fbo instance field private and provide accessor function

commit 600209e5

gl: move gl(base)mixer to library

For use by applications/libraries outside of the opengl plugin.