don't use VBOs (yet) for ptclouds

Use client vertex arrays until we figure out the proper place to delete
GL buffer objects on ptcloud deletion. Performance feels similar.

I looked for how other object types do this cleanup but it seems only
meshes use VBOs via gpu_buffers.c while nearly everything else in all
of blender uses immediate mode. glBegin : glDraw[Array|Element]s ratio
is 484 : 84. Gasp!
add safety to ptcloud_use_color query

Don’t allow drawing with color when there is no color data. These flag
bits should be coupled. PC_DRAW_COLOR will become one of a few enum
values (draw single color, per-point color, false color (various)).