specs/scripts/xml2api: Don't wrap integer array length in quotes.
specs: Aggregate and sort GL_EXT_direct_state_access functions by name.

Otherwise it is impossible to tell which are missing or not, as order
doesn't match Khronos gl.xml's.

(We should probably do this for all extensions too.)

Issue #307.
specs: Add glVertexArrayVertexAttribDivisorEXT.

The only GL_EXT_direct_state_access entry-point that's in Khronos gl.xml
that we were missing AFAICT.

Fixes #307.