Add downlevel capability for MSAA textures

[kvark]

Is your feature request related to a problem? Please describe.
See #2116
In GL backend, we can't create an MSAA-enabled texture if it's not a pure render target.

Describe the solution you'd like
A new downlevel flag to allow that.

Describe alternatives you've considered
none

Additional context

[Wumpf]

For desktop OpenGL we should be able to support it, but there's a different bug here:
The target for tex_storage_2d_multisample here is never called with the right target - can't seen the code picking anywhere TEXTURE_2D_MULTISAMPLE.

This API is available on OpenGL 4.2+ and GLES 3.1+ so this should be supported!

Another bug in there probably: We assume all storage apis are available in 3.0, but this one is not according to https://docs.gl/es3/glTexStorage2DMultisample
In any case the downlevel feature is still needed for WebGL then which only has multisampled renderbuffers https://registry.khronos.org/webgl/specs/latest/2.0/#3.7.6