Add range to anisotropic filter level

[zatherz]

The range is between 1 and 16 (both inclusive), with steps of value 1. The default is still 4.

[Calinou]

Do non-power-of-two values even make sense? This would require custom code to be handled, though.

Perhaps an enum of some sort should be used instead (like for MSAA).

[zatherz]

I thought about that, but the OpenGL spec doesn't seem to force POT, as this is the only search result for power:

It is acceptable for implementation to round 'N' up to the nearest
supported sampling rate. For example an implementation may only
support power-of-two sampling rates.

Although I'm already limiting it to 16 when an implementation could support more than that (only a minimum of 2 is specified for the max), so it might make sense to make it an enum (I even wrote some code for that while fixing this, but ultimately decided to submit this PR). Does anybody else have any inputs?

[zatherz]

Bump