[GRASS5] Display Driver Updates

[Glynn Clements]

I've just committed a substantial restructuring of the display
drivers. The actual code hasn't changed much, but there has been quite
a lot of re-organisation. Please test any d.* modules which you are
familiar with.

The most significant code change is that the driver library no longer
provides a main() function, and doesn't expect the driver to export
specific symbols. Instead, each driver has its own "stub" main
function which calls LIB_main(), passing a table of pointers to the
actual driver functions.

This eliminates the assumptions regarding the behaviour of the linker.
It should be possible to build the driver library as a dynamic library
on all platforms which support dynamic libraries.

I haven't updated the HTMLMAP driver, so it won't build any more.
AFAIK, it wasn't being used due to the fact that d.vect uses
G_plot_area rather than R_polygon_abs, which doesn't work with the
HTMLMAP driver.

-- 
Glynn Clements <glynn at gclements.plus.com>