glbufferpool: add bufferpool options for the various texture targets

glbufferpool: add bufferpool options for the various texture targets

This was chosen over relying solely on the caps as glupload needs to propose an
allocation and set the texture target based on the output caps.  Setting the
caps in the config is currently pointless as they are overwritten in a lot of
element's decide_allocation functions.

This provides a mechanism for the buffer pool to be configured for a certain
texture target when none has been configured.