vbo: fix GL_LINE_LOOP stray line bug

vbo: fix GL_LINE_LOOP stray line bug

When long GL_LINE_LOOP primitives don't fit in one vertex buffer they
have to be split across buffers.  The code to do this was basically correct
but drivers had to pay special attention to the _mesa_prim::begin,end flags
in order to draw the sections of the line loop properly.  Apparently, the
only drivers to do this were those using the old 'tnl' module for software
vertex processing.

Now we convert the split pieces of GL_LINE_LOOP prims into GL_LINE_STRIP
primitives so that drivers don't have to worry about the special begin/end
flags.  The only time a driver will get a GL_LINE_LOOP prim is when the
whole thing fits in one vertex buffer.

Mostly fixes bug 81174, but not completely.  There's another bug somewhere
in the src/gallium/auxiliary/draw/ code.  If the piglit lineloop test is
run with -count 4096, rendering is correct, but with -count 4097 there are
stray lines.  4096 is a magic number in the draw code (search for "4096").

Also note that this does not fix long line loops in display lists.  The
next patch fixes that.

v2: fix incorrect -1 in vbo_compute_max_verts(), per Charmaine.  Remove
incorrect assertion which was added in vbo_copy_vertices().

Bugzilla: https://bugs.freedesktop.org/show_bug.cgi?id=81174
Bugzilla: https://bugs.freedesktop.org/show_bug.cgi?id=49779
Bugzilla: https://bugs.freedesktop.org/show_bug.cgi?id=28130

Reviewed-by: Marek Olšák <marek.olsak@amd.com>
Reviewed-by: Jose Fonseca <jfonseca@vmware.com>
Reviewed-by: Sinclair Yeh <syeh@vmware.com>