Alpha channel refactor

[ucupumar]

From the last discussion in discord, I think alpha channel can become a dedicated channel for more simplicity and flexibility. The implementations could be something like these

• No more alpha toggle on the main color/rgb channel
• Alpha is a special channel that can be paired to a RGB channel, the bake result of these channels will always be merged into one image
• When paired color channel is enabled in a layer, it should automatically also write to alpha channel
• No more background layer since it's mostly to get around the current implementation of alpha
• Simpler card UI element for mask/channel/vector/layer