Canvas mipmap generation doesn't apply to depth buffer

[bjornbytes]

When creating a Canvas with mipmaps = true (the default), textures are created with a full mipmap chain and their mipmaps get generated after rendering to the Canvas. However, the mipmap generation currently doesn't apply to the depth buffer, which is not the intended behavior. Going to be ensuring vulkan work does not have this problem, but this issue can serve to track and call out the bug in case others run into it or want to fix it sooner.

[bjornbytes]

Fixed (in OpenGL).