meta: Always do GenerateMipmaps in linear colorspace.

meta: Always do GenerateMipmaps in linear colorspace.

When generating mipmaps for sRGB textures, force both decode and encode,
so the filtering is done in linear colorspace, regardless of settings.

Fixes a WebGL conformance test in Chrome:
https://www.khronos.org/registry/webgl/sdk/tests/conformance2/textures/misc/tex-srgb-mipmap.html?webglVersion=2

Bugzilla: https://bugs.freedesktop.org/show_bug.cgi?id=97322
Signed-off-by: Kenneth Graunke <kenneth@whitecape.org>
Reviewed-by: Topi Pohjolainen <topi.pohjolainen@intel.com>