WebGLTextures: Fix FramebufferTexture's allocation. (#23854)
WebGLTextures: Fix FramebufferTexture's allocation. (#23854)

There were two bugs in the allocation code:

1. Didn't check if memory needs to be allocated and always used null pixels.
2. Didn't allocate the memory needed for mipmap.
Liuer/three.js,gero3/three.js,jpweeks/three.js,gero3/three.js,mrdoob/three.js,aardgoose/three.js,06wj/three.js,makc/three.js.fork,greggman/three.js,WestLangley/three.js,makc/three.js.fork,donmccurdy/three.js,jpweeks/three.js,Liuer/three.js,aardgoose/three.js,greggman/three.js,06wj/three.js,WestLangley/three.js,mrdoob/three.js,donmccurdy/three.js