Note to myself, next time, better check the fix before pushing it.
GL_ARB_texture_gather is defined if there is support for the extension
not only when the extension is enabled. Do this check ourself with
GPU_ARB_texture_gather define.
Original fix 822de6e9e1
The extension GL_ARB_texture_gather is reported to be supported and does
not trigger an error when enabled but the textureGater functions are not
defined.
Workaround is to disable the use of this extension on such systems.
This commit introduce the computation of a depth pyramid containing min and max depth values of the original depth buffer.
This is useful for Clustered Light Culling but also for raytracing on the depth buffer (SSR).
It's also usefull to have to fetch higher mips in order to improve texture cache usage.
As of now, 1st mip (highest res) is half the resolution of the depth buffer, but everything is already done to be able to make a fullres copy of the depth buffer in the 1st mip instead of downsampling.
Also, the texture used is RG_32F which is a too much but enough to cover the 24bits of the depth buffer. Reducing the texture size would make things quite faster.
