Extend the relaxation on readPixels to the multisample attachments created by core functions of ES 3.0 and later

[dreamelon]

As discussed here

the relaxation in the EXT spec is explicitly limited to attachments created via RenderbufferStorageMultisampleEXT / FramebufferTexture2DMultisampleEXT.

So from spec perspective, implementations should still distinguish whether the multisample attachment was created by:

EXT path → relaxation applies
Core path → no relaxation; GL_INVALID_OPERATION required

I would like to extend the relaxation to core path, would you agree @zmike @janharaldfredriksen-arm