glapi: add GL_EXT_direct_state_access xml

XXX: Since nowadays api_exec.c is autogenerated, it won't
compile for the unimplemented legacy functions. Should those
just be commented out of the XML then?
glapi: add (part of) GL_ARB_separate_shader_objects

The functionality is already implemented in GL_EXT_direct_state_access.

This allows the generated Mesa code api_exec.c to compile
using the unsuffixed names.
glapi: add (part of) GL_ARB_viewport_array

These two entrypoints were also part of GL_EXT_direct_state_access.
This allows usage of the unsuffixed name in Mesa without angering
the auto-generated source.
glapi: remove legacy entrypoints from GL_EXT_direct_state_access

The script-generated api_exec.c don't know about no unimplemented
functions.
glapi: remove unimplemented entrypoints from DSA

None of the extension which these... extend are implemented.

They might be in the future, in which case they can be
added back.