Signs stick to the first 'valid' image

[CvoidWyvern]

When placing down an image, the first time signpic detects that the sign can display something it locks in that image, scale etc, making it so you can't do things like tweak the scale while writing the sign. Even deleting all text on the sign or otherwise making it invalid doesn't stop the image from rendering, but the sign will begin rendering again. Moving outside of render distance makes the signs render as they should as we'd expect it to.

In some cases signpic will consider the sign valid before all the info is added, making it not display anything as it doesn't update from that. This might be the cause of a lot of the difficulty people are having copying in signs.

Tested on latest Signpic on 1.18.1 and 1.18.2, forge 39.1.2 and 40.0.48