Improve memory efficiency and improve conversions

o use KisRandomAccessorNG instead of read/writePlanarBytes
This improves memory efficiecy and it is also much faster/reliable
o simplify KisGmicSimpleConvertor
o add benchmarks/tests related to gmic:
- conversion between pixel formats (gmic->QImage, gmic->KisPaintDevice)
- conversion between Krita colorspace: rgba 8 bit -> rgba 32 bit float
(takes around 10 seconds for 4096x4096 picture)
o whitelist more filters due to this improvements!

Automatic testing:
Curent status of gmic filters in Krita:
- total filters: 260
- known failings (blacklisted filters): 15
- known success: 245