OpenCOlorIO: Add explicit linearSRGB -> sRGB lookup table

Previously converting from linear space to SRGB was doing rather
slow inverted 1D lookup. Adding explicit inverse LUT gives 20%
speedup of OpenGL render.

Next question is: why do we even bother with sRGB conversion here,
OpenGL is already in the proper space so in theory we can avoid
quite some color space conversions. In any case, having this case
optimized in nice anyway.

release/datafiles/colormanagement/config.ocio

@@ -247,6 +247,7 @@ colorspaces:
     allocation: uniform
     allocationvars: [-0.125, 4.875]
     to_reference: !<FileTransform> {src: srgb.spi1d, interpolation: linear}
+    from_reference: !<FileTransform> {src: srgb_inv.spi1d, interpolation: linear}
 
   - !<ColorSpace>
     name: VD16