copyTextureToTexture pixelStorei fix to prevent upside-down texture uploads

[felixpalmer]

The WebGLRenderer.copyTextureToTexture function does not set the pixelStorei parameters for the dstTexture, resulting in incorrect data upload if the pixelStorei parameters have changed after the dstTexture is created and copyTextureToTexture is called. An example would be creating a texture atlas with flipY set to false and then creating another texture with flipY set to true. If after this we try to upload new texture data to the atlas the flipY parameter is be true, and thus our images will appear upside down.

Relevant section of WebGL spec: https://www.khronos.org/registry/webgl/specs/latest/1.0/#PIXEL_STORAGE_PARAMETERS

To demonstrate the issue, take a look at the https://threejs.org/examples/?q=partia#webgl_materials_texture_partialupdate example. It needs a slight modification to draw gradients rather than solid squares so that it is clear when the data is upside down (see change below). While the example is running, change the pixelStorei parameter on the renderer by setting a breakpoint and then executing:

renderer.getContext().pixelStorei(37440, false)

After this all texture uploads will appear upside down. Screenshot demonstrating the output, notice how only some gradients are dark at the bottom.

[felixpalmer]

Should resolve #15590

[Mugen87]

Please undo the modifications to the example unless you can demonstrate the issue without use of the debugger.

Apart from that, the code changes to copyTextureToTexture() seem to work. Although the code could be refactored in the future by consolidating some logic in WebGLTextures.

[mrdoob]

Thanks!