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Ell authored
Add a GimpPaintTool::is_alpha_only() virtual function, which subclasses can override to indicate whether painting only affects the alpha channel (assuming FALSE by default). Override the function in Gimp{PaintBrush,Ink,Clone}Tool, returning TRUE when the current paint mode only affects the alpha (as per gimp_layer_mode_is_alpha_only(); see the previous commit,) and in GimpEraserTool, returning TRUE when the target drawable has an alpha channel. When the function returns TRUE, and the target drawable doesn't have an alpha channel, or the alpha channel is locked, show a BAD cursor modifier, and raise an appropriate warning when attempting to paint.
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