ImageUtils: Always use image/png in getDataURL().

[Mugen87]

Fixes #30326.

Description

This PR makes sure image/png is always used in getDataURL() to avoid quality and transparency issues when serializing large images.

As mentioned in #30326 (comment), the JPG fallback was implemented when texture serialization was initially developed ten years ago. In the meanwhile, hardware should be capable to handle images uniformly as image/png.

[github-actions]

📦 Bundle size

Full ESM build, minified and gzipped.

	Before	After	Diff
WebGL	339.53 79.09	339.53 79.09	+0 B +0 B
WebGPU	493.61 137.21	493.61 137.21	+0 B +0 B
WebGPU Nodes	493.08 137.1	493.08 137.1	+0 B +0 B

🌳 Bundle size after tree-shaking

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

	Before	After	Diff
WebGL	465.41 112.16	465.25 112.11	-153 B -55 B
WebGPU	564.15 152.87	563.99 152.81	-153 B -59 B
WebGPU Nodes	519.53 142.45	519.38 142.39	-153 B -59 B