Tweak libtonemap's CPU interface to support batching.

This is in response to feedback that utilizing the tonemap api to
generate a LUT may require more CPU cycles as common parameters
describing the tone-mapping curve must be regenerated. Support this by
taking in a list of colors, rather than a single color, so that a LUT
can be one-shot computed.

Bug: 200310159
Test: librenderengine_test
Change-Id: I4b9ef8ef6bd95eb25aedd2b16268dc6e58828208