Improve textureSize usage in process material

[QbieShay]

Godot version

4.2beta3

System information

Fedora, RX 590

Issue description

Follow up to #83831

The particle system currently uses textureSize() an unnecessary amount of times and it needs to be optimized.

Steps to reproduce

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Minimal reproduction project

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[clayjohn]

Basically we just need to read textureSize once and store it in a variable that we use later. We can read it at the same time that we initialize all the randomness params.