GL/GLES: Add anisotropic filtering support

[sarbes]

Description

The PR adds support for anisotropic filtering to GL/GLES.

It is an advanced setting for the time being:

<advancedsettings>
  <gui>
    <anisotropicfiltering>16</anisotropicfiltering>
  </gui>
</advancedsettings>

Motivation and context

Texture surfaces at glancing angles are a bit blurry (if sampled with mipmapping enabled) or display shimmering artifacts (without mm).

With Anisotropic filtering, the GPU might sample from multiple locations, giving a nicer looking result. The actual implementation is highly GPU dependent.

How has this been tested?

On my RX570, it has no effect with our current texture pipeline. Forcing mipmaps, there is some visible change on rotated surfaces.

What is the effect on users?

Higher image quality for X/Y rotated surfaces, with possible performance decrease.

Screenshots (if appropriate):

Forced mipmap:

Forced mipmap + anisotropic filtering:

Types of change

• Bug fix (non-breaking change which fixes an issue)
• Clean up (non-breaking change which removes non-working, unmaintained functionality)
• Improvement (non-breaking change which improves existing functionality)
• New feature (non-breaking change which adds functionality)
• Breaking change (fix or feature that will cause existing functionality to change)
• Cosmetic change (non-breaking change that doesn't touch code)
• Student submission (PR was done for educational purposes and will be treated as such)
• None of the above (please explain below)

Checklist:

• My code follows the Code Guidelines of this project
• My change requires a change to the documentation, either Doxygen or wiki
• I have updated the documentation accordingly
• I have read the Contributing document
• I have added tests to cover my change
• All new and existing tests passed

[neo1973]

@sarbes: Please add the new settings to https://kodi.wiki/view/Advancedsettings.xml