mesa: fix incomplete GL_NV_half_float implementation

All of the VertexAttrib* functions were missing.

Fixes: ef66e02a408c ("src/mesa: add GL_NV_half_float extension support (v2)")

Reviewed-by: Marek Olšák <marek.olsak@amd.com>
Part-of: <https://gitlab.freedesktop.org/mesa/mesa/-/merge_requests/10431>
7 files changed