Proposed fix for T101775
The issue is : when trying to filter out keyframes corresponding to animated grease pencil data (not layer keyframes, but data related to grease pencil object, e.g. layer opacity) using the 'Grease Pencil' filter, the keyframes remain visible in the summary.
This is caused by the call to animdata_filter_ds_obdata which creates an item while it should not. This function deals with animated data from all kind of objects, but does not deal properly with grease pencil objects (the case of OB_GPENCIL type object is not handled in the switch statement).
Actually, grease pencil data is already handled by another function : animdata_filter_ds_gpencil, called later in animdata_filter_dopesheet_ob.
So currently, the handling of these data in the summary is done twice.
This patch proposes an easy way of fixing the issue : not calling animdata_filter_ds_obdata in the case of grease pencil object, and leaving the specific 'animdata_filter_ds_gpencil` deal with it.