Add GL_MESA_texture_const_bandwidth

[robclark]

GL_MESA_texture_const_bandwidth adds a way for an app to request the use of a format which does have data dependent bandwidth characteristics, in order to avoid a potential side-channel.

[robclark]

fwiw, original MESA implementation is at https://gitlab.freedesktop.org/mesa/mesa/-/merge_requests/25464

[Triang3l]

The enumeration name finally chosen in Mesa, GL_CONST_BW_TILING_MESA, seems somewhat unusual considering that OpenGL doesn't seem to use such shortenings frequently. _CONSTANT is referenced 27 times in glext.h, while _CONST is used only for CONST_EYE in the ancient GL_NV_texture_shader. The original GL_CONSTANT_BANDWIDTH_TILING_MESA suggestion from that GitLab merge request seems much more consistent with the rest of OpenGL and more intuitive.

[zmike]

WG 12 March 2025: Approved to merge. @robclark should make any final changes then ping me again.

[robclark]

So what is the verdict on naming. I can rename the enums, but renaming the extension would be an ABI break, since this extension has been in mesa for a while.

[zmike]

The verdict is do whatever you want.

[robclark]

The verdict is do whatever you want.

I'd vote to land it as-is, since it has been in mesa for a while now

[zmike]

@oddhack please merge