Allow for sub-classes to change pad templates to
support other texture targets, and bind input textures
accordingly.

When setting the caps, also store the texture target.

By default, glfilter only reports 2D texture targets
in the default caps, but sub-classes can change that
and it would be nice if they could easily find out
which texture targets were negotiated.

This adds 2 fields to the public struct, but since
it's unreleased -base API, it's not an ABI break.