WebGLTexture: Refactored generateMipmaps usage.

[mrdoob]

The current implementation crashed when the code tried to generate mipmaps for a WebGLRenderTarget because rendertarget.texture.image is undefined and the code was looking for rendertarget.texture.image.width.

FYI @takahirox

[takahirox]

Thanks! If we had auto regression test, we could easily find this issue before merging.

[mrdoob]

I heard @fernandojsg is working on it 😊

[mrdoob]

However, the real problem is that WebGLTextures is a mess... 😕

[takahirox]

Agreed. I had a hard time adding maxMipLevel because WebGLTextures is a bit complex. We need WebGLTextures refactoring.