Configure ATI Mach64 for 16 bits-per-pixel ONLY?

[george+freebsd at m5p.com]

I have an ATI Mach64 card in a relatively old machine (more than five years).
Back in the FreeBSD 6.2 days, the X server would run the card in 16-bit-per-
pixel mode with no prompting on my part.

Then I upgraded to FreeBSD 7.0, and X (and the atimisc driver) decided it
would be a good idea to default to 32 bits per pixel.  Due to the oldish
CPU (AMD Athlon(tm) XP 1500+), my video slowed down to a crawl.  But I
put a "DefaultDepth 16" into my xorg.conf and everything was okay again.

Now I have upgraded to FreeBSD 7.2 and Xorg 7.4_1, and the mach64 driver
is once again insisting on providing a 32-bit-per-pixel video mode,
despite everything I am saying in my xorg.conf:

	Section "Device"
		Identifier	"Builtin Default ati Device 0"
		Driver	"mach64"
	EndSection
	Section "Screen"
		Identifier	"Builtin Default ati Screen 0"
		Device	"Builtin Default ati Device 0"
		DefaultDepth 16
		SubSection "Display"
			Depth 16
			Virtual 1440 900
		EndSubSection
	EndSection
	Section "ServerLayout"
		Identifier	"Builtin Default Layout"
		Screen	"Builtin Default ati Screen 0"
	EndSection

xdpyinfo still cheerfully reports seven different depths and bits per
pixel, and the current screen #0 even includes one visual with 32 bits
per pixel.  Once again, my video performance is very sluggish.

Is there a way to persuade this driver that I want 16 bits per pixel
ONLY and no other configuration?  I even made a stab at ripping out
all the code that appeared to support 24 and 32 bits per pixel, to no
avail.                                               -- George Mitchell