stw: add some no-op functions for GL_EXT_dsa, GL_NV_half_float

stw: add some no-op functions for GL_EXT_dsa, GL_NV_half_float

Viewperf 12 calls wglGetProcAddress() to get pointers to some unsupported
DSA and half-float functions.  We return NULL but Viewperf doesn't check
for null before trying to jump through the pointer.  That causes a crash.

This patch adds no-op functions to call instead (used by the next patch).
This avoids the crash but the rendering is incorrect.

Some DSA functions are being added to Mesa at this time so we may be
able to remove some of these no-ops in the future.

More no-op functions may be added as needed.

VMware PR1383421

Reviewed-by: José Fonseca <jfonseca@vmware.com>