mesa: fix spurious wglGetProcAddress / GL_INVALID_OPERATION error

mesa: fix spurious wglGetProcAddress / GL_INVALID_OPERATION error

On Windows, the Piglit primitive-restart test was failing a
glGetError()==0 assertion when it was run w/out any command line
arguments.  Piglit's all.py script only runs primitive-restart
with arguments so this case isn't normally hit during a full
piglit run.

The basic problem is Microsoft's opengl32.dll calls glFlush
from wglGetProcAddress() and Piglit uses wglGetProcAddress() to
resolve glPrimitiveRestartNV() which is called inside glBegin/End.
See comments in the code for more info.

Plus, improve the comments for _mesa_alloc_dispatch_table().

Cc: <mesa-stable@lists.freedesktop.org>
Acked-by: Sinclair Yeh <syeh@vmware.com>