TSL: Refine pow2/3/4 implementation and remove the polyfill for pow on Windows

[shotamatsuda]

Related issue: #31712

Description

This PR changes the implementation of pow2, pow3, and pow4 to be exact powers of 2, 3, and 4 respectively, regardless of the host implementation of pow.

Additionally, it removes the polyfill for pow on Windows, because it is only valid for odd integer exponents, as explained in the aforementioned issue. This does make the behavior of pow with odd integer exponents unstable on Windows, but it is a reasonable behavior and cannot be avoided unless introducing a precise detection of integer exponents in floating-point numbers.

[github-actions]

📦 Bundle size

Full ESM build, minified and gzipped.

	Before	After	Diff
WebGL	338.91 79.1	338.91 79.1	+0 B +0 B
WebGPU	578.73 159.37	577.78 159.15	-942 B -215 B
WebGPU Nodes	577.34 159.13	576.39 158.91	-942 B -215 B

🌳 Bundle size after tree-shaking

Minimal build including a renderer, camera, empty scene, and dependencies.

	Before	After	Diff
WebGL	470.7 113.91	470.7 113.91	+0 B +0 B
WebGPU	649.28 175.4	648.33 175.2	-954 B -204 B
WebGPU Nodes	603.38 164.55	602.42 164.32	-960 B -225 B

[sunag]

/cc @brunosimon