vbo: implement primitive merging for glBegin/End sequences

vbo: implement primitive merging for glBegin/End sequences

A surprising number of apps and benchmarks have poor code like this:

glBegin(GL_LINE_STRIP);
glVertex(v1);
glVertex(v2);
glEnd();
// Possibly some no-op state changes here
glBegin(GL_LINE_STRIP);
glVertex(v3);
glVertex(v4);
glEnd();
// repeat many, many times.

The above sequence can be converted into:

glBegin(GL_LINES);
glVertex(v1);
glVertex(v2);
glVertex(v3);
glVertex(v4);
glEnd();

Similarly for GL_POINTS, GL_TRIANGLES, etc.

Merging was already implemented for GL_QUADS in the display list code.
Now other prim types are handled and it's also done for immediate mode.

In one case:
                                 before   after
-----------------------------------------------
number of st_draw_vbo() calls:     141      45
number of _mesa_prims issued:     7520     632

Reviewed-by: Eric Anholt <eric@anholt.net>
Reviewed-by: José Fonseca <jfonseca@vmware.com>