GLTFExporter: Check for document before creating OffscreenCanvas

[CodyJasonBennett]

Picked from #23998

Related issue: #23995

Description

This PR fixes a regression in GLTFExporter file sizes reported in #23995. It seems as if the OffscreenCanvas.convertToBlob function uses a 100% quality level by default in Blink powered browsers.

To fix this I've made two changes.

Check for the existence of document to identify if we are currently in a worker context. If we are, check for the existence of OffscreenCanvas and use it if it exists. If both are false, use the canvas API.
Infer default quality levels to pass to OffscreenCanvas.convertToBlob based on the given mimeType. Based on Blink's code the [default JPEG quality level](https://source.chromium.org/chromium/chromium/src/+/main:third_party/blink/renderer/platform/image-encoders/image_encoder.cc;l=85;drc=52f06e6b43ff95eccf79e0a5df8d4d83c029130a;bpv=0;bpt=1) should be 92% and the [default WebP quality](https://source.chromium.org/chromium/chromium/src/+/main:third_party/blink/renderer/platform/image-encoders/image_encoder.cc;l=100;drc=52f06e6b43ff95eccf79e0a5df8d4d83c029130a;bpv=0;bpt=1) should be 80%.

This contribution is funded by Matrix.org

Related: #24031

[mrdoob]

Thanks!