Since some tools share same brush fields in `toolsettings`, the tool can
get overwritten when exiting fullscreen and multiple `ScrArea`s having
different brush settings (like a image editor space could have different
texture paint tool than what is selected in the 3d viewport in texture
paint mode).
This patch fixes the problem by putting the original`ScrArea` to the
last in `screen->areabase`, so when exiting fullscreen, the tool gets
preserved since it's always the last one to be set.
YimingWu