Avoid generating rasterized image when copying large items

[lynzrand]

This is not a issue on Beta versions alone, but I think it fits here.

Affinity Designer has a very handy feature of generating a bitmap version of the selection when copying. But when copying large items, like something larger than 10000x10000 px, this could lead to the program freezing for many seconds (to generate the rasterized version), and even crashing (when the expected image size is larger than system memory size).

Expected solution: one of following

• Add a configuration to avoid rasterization when item size is larger than a user-entered value
• Warn user about large items when copying, and add an option in the warning to copy without rasterizing it
• Move rasterization feature to a separate shortcut and menu option, like "Copy PNG (Ctrl+Shift+C)"