fix brushstroke prediction

[Hiro Lynx]

• very shaky in some cases, like with finger drawing
• completely bogus when more than one finger is registered
• it was planned that the brushstroke prediction would be blended in-shader with the current working texture instead of blended by the GPU during a subsequent draw call, so it would look better in some cases like partial or dynamic opacity
• prediction being drawn outside active selection while it should not