Add note about ignored texture properties for ImageBitmap to ImageBitmapLoader document

[takahirox]

.flipY and .premultiplyAlpha of Texture properties with ImageBitmap are ignored because UNPACK_FLIP_Y_WEBGL, UNPACK_PREMULTIPLY_ALPHA_WEBGL (, and UNPACK_COLORSPACE_CONVERSION_WEBGL) for gl.pixelStorei are ignored if the image source is ImageBitmap.

https://www.khronos.org/registry/webgl/specs/latest/1.0/#6.10

I'd like to add the note about it to ImageBitmapLoader document. Please feel free to correct the sentences if looking weird because English isn't my first language.

[Mugen87]

I would shortly note why both properties are ignored (because unlike normal images they are needed when the bitmap is created; not at texture upload time).

[takahirox]

So, how about adding

... are ignored because corresponding UNPACK_FLIP_Y_WEBGL and UNPACK_PREMULTIPLY_ALPHA_WEBGL parameters for gl.pixelStorei() are ignored if image source is ImageBitmap.

[Mugen87]

It seems more important to me to differentiate between creation and upload time of the texture. In context of ImageBitmap, you need these configuration parameters when creating the bitmap. In all other cases, you need them when the texture is uploaded to the GPU. That's the important bit users should aware of and the reason why setting Texture.flipY and Texture.premultiplyAlpha after the creation process are ignored.

[takahirox]

How about this

Note that Texture.flipY and Texture.premultiplyAlpha with ImageBitmap are ignored. ImageBitmap needs these configuration on bitmap creation unlike regular images need them on uploading to GPU. You need to set the equivalent options via ImageBitmapLoader.setOptions() instead. Refer to WebGL specification for the detail.

[Mugen87]

Sounds good 👍!

[takahirox]

Updated.

[mrdoob]

Thanks!