Factored out generic LINE implementation based on SETPIXEL from the three drivers...

Factored out generic LINE implementation based on SETPIXEL from the three drivers previously containing the identical code.

Note: Originally I planned to create a module to be linked to the driver. However this turned out to cause quite some issues with the Makefiles - especially on the Apple2 where one driver uses to generic LINE implementation and the other doesn't. The final Makefile was so ugly that I rather opted for including the code in question on source level. In order to avoid to unwanted cross dependencies the whole .inc file content is wrapped in its own scope.

git-svn-id: svn://svn.cc65.org/cc65/trunk@4553 b7a2c559-68d2-44c3-8de9-860c34a00d81