Inconsistent texture binding + account for depth in generateMipMap

[mbredif]

Problem:

• glTextureType is bound and set up.
• _gl.TEXTURE_2D is used for mipmap generation and unbound.
  however glTextureType may not be _gl.TEXTURE_2D

three.js/src/renderers/webgl/WebGLTextures.js

Lines 1227 to 1237 in e8877df

	state.bindTexture( glTextureType, textureProperties.__webglTexture );
	setTextureParameters( glTextureType, texture, supportsMips );
	setupFrameBufferTexture( renderTargetProperties.__webglFramebuffer, renderTarget, texture, _gl.COLOR_ATTACHMENT0, glTextureType );
	if ( textureNeedsGenerateMipmaps( texture, supportsMips ) ) {
	generateMipmap( _gl.TEXTURE_2D, texture, renderTarget.width, renderTarget.height );
	}
	state.bindTexture( _gl.TEXTURE_2D, null );

fix :

• use glTextureType instead of _gl.TEXTURE_2D
• adapt generateMipMap to take depth into account

three.js/src/renderers/webgl/WebGLTextures.js

Line 131 in e8877df

textureProperties.__maxMipLevel = Math.log2( Math.max( width, height ) );