Fix multimesh memory leak and edge cases

[beicause]

Fixes:

• Indirect draw command buffer memory leak
• GPU out of memory if use_indirect=true and multimesh_set_visible_instances(rid, -1)
• GPU out of memory if multimesh_allocate_data(rid, 1) and then multimesh_allocate_data(rid, -1)

And:

• Simplify setting vertex count in the command buffer (follow-up to the 2nd in 99455#comment)
• Follow the visible instance count rather than making all instances invisible, after setting a mesh with indirect drawing