GL: Work around drivers that generate mipmaps in linear color space Mac drivers generate mipmaps in linear color space. To work around this, copy the sRGB texture to a linear texture, generate mipmaps and then copy back. TEST=conformance2/textures/misc/tex-srgb-mipmap.html BUG=angleproject:4646 Change-Id: I8675d0ab004bcd2985f685d64cbb84deff5f1c86 Reviewed-on: https://chromium-review.googlesource.com/c/angle/angle/+/2211083 Reviewed-by: Jonah Ryan-Davis <jonahr@google.com> Reviewed-by: Geoff Lang <geofflang@chromium.org> Commit-Queue: Geoff Lang <geofflang@chromium.org>