Add CPU implementation for tone-mapping curves.

This allows for library implementations that do not wish to place a
dependency on a GPU driver to build a lookup table.

A secondary use-case, which is included in this CL, is to allow for
building unit-tests for checking the validity of the tone-mapping curve.
See the newly added test in RenderEngineTest which validates the PQ
tone-mapping curve by checking grey values.

Bug: 200310159
Test: librenderengine_test
Change-Id: Ic765485c22c53b4dc58a2bc8db42fd51ac7f2eea