This change deprecates rr::Rcp_pp with rr::Rcp, which makes sure to
correctly compute the reciprocal using the Newton-Rhapson refinement
only if the current target supports the required instrinsic, otherwise
using 1 / x. Currently, only LLVM on Intel will use NR. Note that
passing in Precision::Relaxed will produce a faster, but less precise
reciprocal.

Also removed PixelProgram::linearToSRGB as it's unused.

Bug: b/169760262
Bug: b/149574741
Change-Id: I4a2f943aa60116c4397d7a8ae18583a260824788
Reviewed-on: https://swiftshader-review.googlesource.com/c/SwiftShader/+/50648Reviewed-by: Alexis Hétu <sugoi@google.com>
Reviewed-by: Nicolas Capens <nicolascapens@google.com>
Tested-by: Antonio Maiorano <amaiorano@google.com>
Commit-Queue: Antonio Maiorano <amaiorano@google.com>