renderer (generic): reset buffer size on grid resize

[mitchellh]

#10354 introduced a heuristic to free buffers when the actual data used is 4x smaller than the capacity.

I don't like this, but its merged as a band-aid.

I think it'd be cleaner instead to always reset our buffer sizes (at least for cells) on frame resize.

We may want to rename our sync to syncRetainCapacity and also have a sync that always exact-fits the data, because I think things like uniforms and image buffers will always want to just exact-size the buffer since they aren't super volatile.