ubb-vga: housekeeping

- ubb-vga.c: added "friendly" 640x480 mode based on
  http://faculty.lasierra.edu/~ehwang/public/mypublications/VGA Monitor Controller.pdf
- README: added regressions and a new challenge

ubb-vga/README

@@ -91,3 +91,7 @@ To do
 - read/write raw frame buffer
 - consider prefetching first word of each line, to make DRAM controller
   open the row and reduce DMA startup latency
+- add option to not disable the LCD (works with some modes if reducing
+  the LCD controller's burst size)
+- find out why 640x480 has so much FIFO noise
+- fix modes 800x600/54 and 800x600/72, broken by the timer change

ubb-vga/ubb-vga.c

@@ -51,6 +51,7 @@ static int bad;
 static const struct mode mode_db[] = {
 /* name		xres yres clkdiv vfront  hsync     hback     htotal	*/
 /*			      vsync  vback				*/
+{ "640x480",	 640, 480, 12, 2, 32, 14, US(2.80), US(1.3), US(32.1) },
 { "640x480/58",	 640, 480, 12, 2, 10, 33, US(3.81), US(1.91), US(32.7) },
 { "640x480/70",	 640, 480,  9, 2,  8, 29, US(1.90), US(2.06), US(24.8) },
 
@@ -68,6 +69,10 @@ static const struct mode mode_db[] = {
 };
 
 /*
+ * 640x480: loosely based on the timing from "Build a VGA Monitor
+ * Controller" by Enoch Hwang
+ *
+ * 
  * 640x480/58: http://tinyvga.com/vga-timing/640x480@60Hz
  *
  * H: 3.81+0.64+25.42+1.91 us = 31.78 us