Blender Git Loki

Tweak 2D line dashed shader to take a scale parameter.

Even though in some cases this does not seems useful, in others (like
zommed 2D views) we have to correct MVP matrix scaling to get fixed
dashes size.

Note that we could do that differently (commented about it in shader),
would also have been cleaner to extract that MVP scale from within the
shader, but there does not seem to be a way to initialize uniform values
from within a shader, and would rather avoid recomputing the scale for
every run (especially since 3DViewport does not need it e.g.).

Part of D2647.