Fix pixel color calculation for different data formats

[mishpajz]

Change the way pixel data is obtained from image in dominantColorFrequencies.

Previously, raw image data was retrieved from CGImage and assumed to be in a RGBA8 format (meaning each pixel has 4 bytes, with R, G, B and A values in this order). However, this is not guaranteed, and if the image was in a different format, the algorithm returned wrong values.

This PR adds conversion from CGImage to CIImage, and uses CIContext to render the image to a bitmap buffer. This ensures that the image data is always in RGBA8 format.