Add copyTextureToTexture3D() to WebGLRenderer

[DavidPeicho]

Description

This PR adds the copyTextureToTexture3D() to the class WebGLRenderer.

Why

Just like for 2D textures, it can be useful to update only a small part of a texture on the GPU. When dealing with volumes of consequent size ( > 128 x 128 x 128 for instance), being able to re-upload only what changed gives enormous performance improvement.

Improvements

Right now, the pixelStorei parameters aren't reset:

_gl.pixelStorei( _gl.UNPACK_ROW_LENGTH, width );
_gl.pixelStorei( _gl.UNPACK_IMAGE_HEIGHT, height );
_gl.pixelStorei( _gl.UNPACK_SKIP_PIXELS, sourceBox.min.x );
_gl.pixelStorei( _gl.UNPACK_SKIP_ROWS, sourceBox.min.y );
_gl.pixelStorei( _gl.UNPACK_SKIP_IMAGES, sourceBox.min.z );

If we plan on adding those on the texture object, then it could make sense to do the pixelStorei of those constants directly in WebGLTextures just like it's done for 2D textures. Otherwise, we can just reset those to the default values after the texSubImage3D call.

[mrdoob]

@DavidPeicho Could you also add an example in this PR?

[Mugen87]

It would also be good to update the documentation page of WebGLRenderer.

[DavidPeicho]

Trying to think what good example I could do. I can do a dummy one with colors but that's not fancy.

Just for debugging purposes, on my side I made one updating the "cloud" example with smaller clouds, but that's ugly because it creates discontinuities

[DavidPeicho]

Regarding the pixelStore info, do you prefer new params on texture on simply resetting them after the copy call?

[mrdoob]

Trying to think what good example I could do. I can do a dummy one with colors but that's not fancy.

Don't worry about making it fancy. Once it's working I can try to come up with something 👍

Regarding the pixelStore info, do you prefer new params on texture on simply resetting them after the copy call?

Lets just reset them for now.

[DavidPeicho]

@mrdoob done. Test example is webgl2_materials_texture3d_partialupdate.html.
The test has some random which makes it bad for the E2E tests.

And thanks to the person that wrote the cloud example, this is what I re-used 😄

[mrdoob]

Thanks!

[DavidPeicho]

Thanks!!! ❤️