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/+/2211083Reviewed-by: Jonah Ryan-Davis <jonahr@google.com>
Reviewed-by: Geoff Lang <geofflang@chromium.org>
Commit-Queue: Geoff Lang <geofflang@chromium.org>