New format conversion classes and routines

- A new routine has been added to convert from any component of
  8bit sRGB data to 8bit linear RGB, using a precomputed array.
- Two new classes have been added to easily convert to and from
  the RGB9E5 format and the R11G11B10F format.

Change-Id: I85ca58bed30bcd5a9130bca5040d351badabb19e
Reviewed-on: https://swiftshader-review.googlesource.com/3990
Tested-by: Alexis Hétu <sugoi@google.com>
Reviewed-by: Nicolas Capens <capn@google.com>