Avoid allocation per draw

[kvark]

gfx-rs itself doesn't use any allocations on a call-by-call basis, and it would be great if gfx_text didn't impose this overhead either. The allocations come from the constructed meshes and batches, even if implicit. What can be done:

• have an OwnedBatch created once on init
• update its Slice with updating the vertex/index buffers that the batch uses
• send it to a stream as usual

[Kagami]

:+1: would be nice to have.