Instead of relying on color block names to get the transfer function
intention regarding encoding pixel's luminance, define supported
Electro-Optical Transfer Functions (EOTFs) and inverse EOTFs, that
includes pure gamma or standardized transfer functions.

v3:
- squash linear and unity TFs to identity (Pekka)
- define the right TFs for BT.709 (Pekka and Harry)
- add comment about AMD TF coefficients
Suggested-by: Harry Wentland <harry.wentland@amd.com>
Reviewed-by: Harry Wentland <harry.wentland@amd.com>
Signed-off-by: Melissa Wen <mwen@igalia.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>