add random choose pixel mode

[clansty]

添加了一个随机选择要涂色像素的选择模式

关于为什么需要这个功能，我觉得有以下两个原因：

1. 在操作比较快的时候，涂色完可能无法正常刷新区块，导致下一次涂色选择了刚刚涂过的区域，这样会造成涂色机会的浪费。而随机在未填充区域中选择的话，在遇到未正常刷新的情况下选到相同像素的概率更低，能大大减少涂色机会的浪费
2. 如果从上往下完成作品，那下面会在制作过程中留有大量空白，别人的作品可能会占据你原本规划的位置。如果随机填色的话，整个作品看起来是从模糊到清晰的，大家也可以更早的看到你在画的东西的全貌

Added a selection mode that randomly picks pixels to be colored.

As for why this feature is needed, I think there are two main reasons:

1. When operating quickly, the canvas may not refresh properly after coloring, causing the next coloring attempt to select the same area that was just filled. This wastes coloring opportunities. By randomly selecting from unfilled pixels instead, the probability of hitting the same pixel in such cases is much lower, which greatly reduces wasted coloring attempts.

2. If you complete a work from top to bottom, a large blank area will remain at the bottom during the process, and other users’ works may occupy the space you originally planned. With random coloring, however, the artwork gradually transitions from blurry to clear, allowing everyone to see the overall picture you’re creating much earlier.

[Akroshi]

this would be better if the auto select queued all pixel placements potential at once and then as you print on each account it slowly takes away from the auto selected removing the issue of the current maps data being stale after a failed fetch