fix alpha encoding when using grid workaround

[itaibenari]

when using the grid workaround the alpha auxl item refers to the image before it was wrapped in a grid (image_id) and causes alpha to be lost. so I changed the order and build the alpha layer after the main image.

additionally, for small images (64px), there's an assertion that breaks at heif_encoder_x265.cc:769, hence changed the alpha layer to be monochrome and only use Y.

[farindk]

@itaibenari I think your first patch 8e930c2 is fine (if the alpha is encoded as monochrome and hence can have odd size). The second patch (5866f1a) is problematic because it appears to disable the whole 'clap' processing. This would break any correct codec just to match the error is macOS.

Do we have any example case why we need to modify the 'clap' processing?

[farindk]

Seems to be obsolet now.