macOS: Avoid automatic resizing of layers by fixing them to top-left

When a layer is resized, e.g. during a window resize, the contents of
the layer may lag behind if the client doesn't fill the layer in
response to the window resize and corresponding expose event.

The default behavior is for Core Animation to stretch the content
to fill the layer, but this results in the content "jumping" back
and forth when the content then picks up the new size and fills
the layer.

Instead we tell Core Animation to fix the content to the top left
corner. If a layer is sized up without a corresponding layer contents
update this will result in missing/transparent pixels in the bottom
or right part of the layer, explicitly showing what the result of
the missing paint is. During debugging we also highlight this area
by adding a magenta background color to the layer.

Conversely, if the layer is sized down we don't need to resize it,
we can just keep the fixed top left position, and the content will
stay in place during the resize. This allows for optimizations
during window resizing, where we don't need to allocate new
buffers if the old buffer is larger than the new one.

Change-Id: I265b57e3a0ddff8bbcda3af5d670cd8c3b00b181
Reviewed-by: Tor Arne Vestbø <tor.arne.vestbo@qt.io>

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