Add Chips & Technologies 82C425 CGA LCD/CRT controller emulation

The 82C425 is a CGA-compatible display controller chip. On top of being
able to drive a regular CRT display like an ordinary CGA card, it can
be configured to drive a monochrome 640x200 LCD panel instead.

The chip along with a LCD panel are notably used in the Victor V86P
laptop comupter.

When driving a monochrome LCD, the controller is able to employ some clever
tricks to compensate for he lack of color: by alternately turning dots on
and off with various duty cycles it can achieve displaying 4 or 8 shades
of gray. It can also enhance contrast between the text glyphs and their
background when it's less than the configured minimum (with "SMARTMAP"
algorithm).

The emulation is fairly complete. The 320x200 graphical mode uses 4 gray
shades along with stretching the pixels horziontally much like the real
hardware would. SMARTMAP is implemented for text mode and also matches
the real hardware pretty closely.

The missing bits are:

 * Configurable blink rates
 * Mapping the character map into host address space

The code is based on the T1000 display controller emulation and
still bears strong resemblance to it.

src/include/86box/video.h

@@ -249,6 +249,9 @@ extern const device_t compaq_cga_2_device;
 extern const device_t ogc_device;
 extern const device_t ogc_m24_device;
 
+/* Chips & Technologies 82C425 */
+extern const device_t f82c425_video_device;
+
 /* NCR NGA */
 extern const device_t nga_device;