GL Backend: trying to render into mipmap level other than 0 results in incomplete framebuffer.

[floooh]

Just putting part of #912 into a new ticket:

When creating a GL render target texture with mipmaps, GL_TEXTURE_MAX_LEVEL must be initialized, otherwise trying to render into mipmap levels other then 0 results in an incomplete framebuffer.

(apparently calling glTexImage2D() with a null data ptr doesn't actually reserve storage for that mip level)