vhpidirect: add 'arrays/matrices/framebuffer'

[umarcor]

This PR is work in progress to extend example 'matrices' with an RGB24 framebuffer. A 2D array of integers is allocated and used in VHDL to represent a virtual screen. The buffer can be either saved to PNGs/GIF or shown in an X11 window in "real-time".

A subexample is included where a synthesisable VGA pattern generator is instantiated in a testbench along with a "Virtual VGA screen" module. The virtual screen captures the VGA signals (hsync, vsync, R, G and B), a draws the frames using one of the aforementioned methods. See the figure and explanations added to the docs.

---

To run all the subexamples locally, packages imagemagick and libx11-dev are required on Debian. On Windows, x11docker can be used along with a docker container:

x11docker --runx --no-auth --user=0 -i -- -v /$(pwd)://src -- ghdl/ext bash

---

These sources are inspired by http://ygdes.com/GHDL/fb/ and hackfin/ghdlex.

---

Pending enhancements:

• Implement integer_to_raw24 as a byte-wise operation, instead of decomposing the integer through arithmetic.
• Use a generic package, instead of hardcoding the size of the screen. cinterface/accarraygen example (Bounded by Generic Package - VHDL 2008) umarcor/ghdl-cosim#5.

Future work/ideas: #22