Disable stroke while panning or zooming

[Zacrain]

Closes #301 .

Branched off main. Tested in debug and Windows release export.

Notable changes and explanation of new behaviour:

• While panning or zooming drawing will no longer be possible.
• The brush tool has a special behaviour: when starting to draw with the brush, then starting to zoom, the brush stroke is ended.
• No matter the tool, if you start drawing and then pan while drawing, you can continue drawing smoothly from where you left off once you stop panning.
• Except for the brush tool, geometric shapes can be adjusted while zooming after they have been initiated.

Observed bugs:

• For some reason my graphic tablet's pen movement is not recognized. That seems to be an issue with the main branch though and was not introduced by me.
• Sometimes, while drawing, panning and or zooming, the cursor might disappear / become invisible. It will reappear upon drawing somewhere again. I was not able to reproduce this issue reliably though.

[Zacrain]

Remark: I made a small mistake in the commit message. I've written "draw button", but just meant "drawing". Idk where the button came from. Sry.

[mbrlabs]

Sometimes, while drawing, panning and or zooming, the cursor might disappear / become invisible. It will reappear upon drawing somewhere again. I was not able to reproduce this issue reliably though.

Yeah, that's a bug i'm aware of. It has to do with how the app handles input focus. It manually disables/enables the canvas based on where the mouse is and other input events. It's quite messy and needs a rework honestly :sweat_smile: