Bug 748749 - picked colors don't match image colors...

...when a color profile is active

This commit adds more (still unused) infrastructure to fix this bug:

Ee now keep around color transforms from layer pixels to "R'G'B'A
double" (which is GimpRGB's format) and back. Also add utility
function gimp_image_color_profile_pixel_to_srgb() which converts a
picked pixel to GimpRGB, using the cached color transform.