glfilter: Remove hard-coded pad templates

glfilter: Remove hard-coded pad templates

Add a function to install the default RGBA pad templates,
but don't make them required so that there can be
GstGLFilter sub-classes with different input/output
caps if they want. Remove the hard-coded RGBA restriction in
the set_caps_features call, as it will be taken care
of by intersecting with the pad templates.

Update all the sub-classes to match