Add handling of alpha channel on image load/save and in shader

[MagisterX]

• Correctly handle alpha when loading image
• Preserve alpha when saving image in formats that support it
• Tweaks in shader to correctly display alpha
• Added Checherboard pattern in shader for alpha
• Added two registry entries and checkboxes in settings for displaying Alpha and Checkerboard
• Added separate shader buffer for Gamut Graph to prevent it from being affected by alpha