Geometry Nodes: Improve point instance node performance

This commit uses two changes to improve the performance of the point
instance node.

**Prevent Reallocations**
At 64 bytes, the transform matrix for every instance is rather large,
so reallocating the vector as it grows can become a performance bottle-
neck. This commit reserves memory for the instances that will be added
to prevent unecessary reallocations as the instance vector grows.

In a test with 4 million instances of 3 objects in a collection, the
node was about 40% faster, from 370ms to 270ms for the node.

**Parallelization**
Currently the instances are added by appending to a vector. By changing
this slightly to fill indices instead, we can parallelize the operation
so that multiple threads can fill data at the same time. Tested on a
Ryzen 3700x, this reduced the runtime from the above 270ms to 44ms
average, bringing the total speedup to ~8x.

Note that displaying the instances in the viewport is still much slower
than the calculations in node, this change doesn't affect that.

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