piglit-dispatch: update gl.spec, gl.tm and enumext.spec from the registry

Added GLDEBUGPROC typedef to piglit-dispatch.h.

This update is required for the ARB_texture_buffer_range extension tests
to get the definition of the TexBufferRange function.

EDITs of gl.spec: Fixed alias of GenVertexArraysAPPLE to be
GenVertexArrays instead of GenVertexArray (singular).  (Fix already
present in piglit sources).

Removed GetTransformFeedbackVarying and TransformFeedbackVaryings aliases
in gl.spec from GetTransformFeedbackVaryingNV and TransformFeedbackVaryingsNV
(different signatures).  (Fix already present in piglit sourcres).

Made varyings parameter to TransformFeedbackVaryingsEXT const to match the
non-EXT version and registry/specs/EXT/transform_feedback.txt.

Added constcharPointerARB to gl.tm and used this in ShaderSourceARB to
match constness of glShaderSource.

Made indices pointer in MultiDrawElementsEXT const to match
MultiDrawElements and registry/specs/EXT/multi_draw_arrays.txt.

v2 (Paul Berry <stereotype441@gmail.com>): restore fixes from commits
6409a67 and e9db1f0.