fix(webgl): premultiply straight bitmaps when device ignores createImageBitmap option

[chiefcll]

Summary

Fixes edge ghosting on transparent images on older Safari/WebKit devices that silently ignore the createImageBitmap premultiplyAlpha: 'premultiply' option. Those devices return straight (non-premultiplied) bitmaps, but the WebGL upload hardcoded UNPACK_PREMULTIPLY_ALPHA_WEBGL = false, so the straight pixels were treated as premultiplied — producing the characteristic halo/ghosting around transparent edges.

What changed

• Unified semantics: TextureData.premultiplyAlpha now consistently means "WebGL should premultiply this source on upload." Each loader path (main-thread bitmap, worker bitmap, basic path, HTMLImageElement fallback, SVG bitmap vs SVG ImageData fallback) sets this intent correctly. The WebGL upload trusts that single flag instead of the old isImageBitmap ? false special-case.
• GL fallback: when the device ignores the option, request a straight ('none') bitmap and let WebGL premultiply on upload instead.
• Startup probe (detectPremultiplyAlphaHonored): builds a known straight ImageData, runs it through createImageBitmap({premultiplyAlpha:'premultiply'}), uploads with UNPACK_PREMULTIPLY_ALPHA_WEBGL=false, and reads back via a framebuffer to detect whether the option was actually honored. Honored → red reads back ~128; ignored → ~255.
• premultiplyAlphaHonored config on RendererMainSettings — boolean | 'auto':
  • omitted (undefined) → true: assume honored, no probe (the default; preserves existing behavior with zero overhead)
  • 'auto' → run the startup probe (auto-detect)
  • boolean → force the value and skip the probe (useful for known fleet devices)
• Example: examples/tests/premultiply-alpha.ts renders transparent rocko.png over white/black panels in a 2×2 grid to visualize correct vs ghosted output, with an on-screen readout of the detected premultiplyHonored.

Reviewer notes

• Default is a no-op: with the setting omitted, the fix assumes the option is honored and runs no probe, so behavior is unchanged out of the box. Affected fleets opt in with premultiplyAlphaHonored: 'auto' (detect) or a forced false.
• The probe is cheap (one 1×1 texture upload + framebuffer readback) and fails safe: any error (no WebGL, ImageData unsupported, incomplete framebuffer) resolves to null, leaving the original behavior unchanged. Rendering only differs when the probe (or a forced config value) reports premultiplyHonored === false.
• WebGL-only fix; the Canvas2D backend is intentionally untouched.
• The image worker capability flags are injected via string replacement, so the worker code remains ES5-valid.
• Unit tests cover detectPremultiplyAlphaHonored (honored / ignored / null branches + GL-premultiply-off during the probe). Note CI's visual snapshot runs on chromium, which honors the option — the non-honored upload path is exercised by unit logic, not the visual suite.

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