shaped-texture: Remove mipmap framerate limiting

Although mipmapping is still slower than not mipmapping, commit 16fa2100 simplified N synchronous draw calls per texture tower into just one. So it's more efficient now, and four years have passed since the throttling was introduced so people also have better hardware as well as mutter being generally faster than it used to be. So I am happy to effectively revert commit c9c32835.

This means antialiasing will remain consistent rather than popping in and out of existence.

Closes: #403 (closed)

Warning - This won't look right without !2537 (merged).