Add a "count" attribute to "enums" elements to set the default count

Add a "count" attribute to "enums" elements to set the default count
used for "size" sub-elements.  In the future the "count" attribute may
be removed completely from "size" sub-elements, so gl_API.xml was also
updated.

Support was added for a (currently unused) "mode" attribute for "size"
elements.  Basically, functions are marked as either "get" or "set".  This
will be used in generating size functions for the server-side (where the
"get" functions have to know how much data to return).  It could also be
used to help generate code for src/mesa/main/get.c.