Clamp textures with alpha to transparent color

[sarbes]

Description

On OpenGL, textures are usually clamped to the edge. If a texture has no white space around and the texture is not sampled in a "pixel perfect" manner, outside texels in the clamped portion have some influence on the pixel.

Motivation and context

This PR changes the clamping behavior to GL_CLAMP_TO_BORDER on systems which support it.

How has this been tested?

The OpenGL part looks fine. I would like to get some feedback for GLES.

What is the effect on users?

Better behaving textures containing an alpha layer without white space.

Screenshots (if appropriate):

The texture (zoomed in). Note that three sides of the circle are touching the border:

The affected regions are marked in red (zoomed in):

Before the patch:

After the patch:

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)
• 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

[sarbes]

Obviously, one could debate about this being an issue of the asset. But we don't have GUI elements which rely on the edge clamping behavior, so it should be fine to do.

[sarbes]

Another try. TVOS was not defining the parameters, so I've added that. If the platform does not support it, it shouldn't get past the extension check.

Or should I ifdef TVOS out?

[jenkins4kodi]

I've made some formatting changes to meet the current code style. The diffs are available in the following links:

• PR21549-0001-9e0d994c8b-Clamp-textures-with-alpha-to-trans.diff

For more information please see our current code style guidelines.

[sarbes]

After thinking about it a bit more, this approach is not viable, as edge clamping might be desired in case of gradients or other effects.

The easiest way to fix this would be on content level.