Improve the perfromace of tile canvas

[CaviarChen]

• have a way to figure out outdated canvas when the fow data is modified. #9
• have a faster way to check whether a canvas only contains fog.
• for fog only canvas, a 1x1 HTMLCanvasElement should be enough. #37
• for all fog only canvases, share the same canvas (require copy on write when they have data later) #37
• for all fog only canvases, share the same texture2d cache. #37
• increase the tilecanvas cache. Maybe consider having another canvas cache.