New Color Pipeline workflow and corrections to Alpha Premultiply

[DavidBluecame]

Initial changes to try to fix the (apparently broken) YafaRay color pipeline workflow and to replace the simple gamma input/output correction with a proper sRGB decoding/coding in non-HDR files.

Also some changes to Alpha Premultiply option, that was orphaned in the Exporter a long time ago.

More information here: http://www.yafaray.org/node/670 http://yafaray.org/node/682

• Improved Blender Color Space integration.
• The "simple" gamma correction has been replaced by Color Spaces:
  • LinearRGB Linear values, no gamma correction
  • sRGB sRGB encoding/decoding
  • XYZ XYZ (very experimental) support
  • Raw_Manual_Gamma Raw linear values that allow to set a simple gamma output correction manually
• Fixed: Double application of input gamma to the Blender Color picker. So now scenes will look brighter in general, but they should also look more realistic with less tweaking.
• Gamma input correction no longer used. The color picker floating point color values will be considered already linear and no conversion applied to them.
• For textures, added specific per-texture Color Space and gamma parameters.
• The color values exported to the XML file will be encoded acording to Blender Output Device Color Space setting.
• In yafaray-xml, new commandline option added: "-ics" or "--input-color-space" that allows to select how to interpret the XML color values. By default, for backwards compatibility, color values will be read as "LinearRGB", but using "-ics sRGB", the color values will be interpreted as sRGB. This setting does not affect the textures, as they have already per-texture specific color space/gamma parameters.
• Fixed: when exporting to file there was an error in Blender while reopening it to be shown in the Blender image view.
• Blender expects an already premultiplied output from YafaRay, but if premultiply was disabled in an older version of YafaRay, it's no longer possible to enable it in recent versions of the Exporter, causing wrong renders into Blender.
• Now, premultiply is always forced to "True" when exporting into Blender
• Also, when exporting into File or XML, a checkbox next to the output file name is shown to allow enabling or disabling Premultiply Alpha, giving more choices to the users depending whether they want their output images to be premultiplied or not

[DavidBluecame]

No need for this anymore, too many changes already since this pull request. We will integrate these changes via the new experimental branch.