Full support for gamma correct blending of brushes and gradients

[NotMyFault]

Has been requested before for V1.

Based on intense discussion Affinity needs to improve handling of gamma blending for RGB documents:

• RGB/8 and RGB/16 use gamma 2.2 by default for layer blending, and inside layers brush painting and gradients
• This is physically wrong, blending must be done at gamma 1.0 or linear gamma.
• only for backward compatibility it might be useful to have the option when opening existing files.
• Affinity should use gamma 1.0 as new default for all layer types. Currently users must change default, so most of them forget this.
• Affinity should use gamma 1.0 for pixel brushes. Currently there is no way to change this.
  • workaround: separate brush strokes into own layers, which adds complexity and workload.
• Affinity should use gamma 1.0 for gradient rendering. Currently there is no way to change this. 
  • Workaround: don’t use gradients, instead copy layers and stich from solid colors with transparency. Adds  too much effort in most cases.
• Handling of image formats with LOG or PQ transfer function. Currently forced converted to RGB/32 with many drawbacks (storage, some blend modes and filters don’t work, more complex UI incl. 32 bit preview. Affinity should allow editing in RGB/16, unrelated to transfer function.