Arwinss: screen resolution is partially hardcoded to 800x600.

Tested in 46872.
If you set the screen resolution to 1024x768 you see a black border beyond the 800x600 limit, moreover if you try to set it to 640x480 ros stops with an assert after rebooting:

(subsystems\win32\win32k\main\csr.c:48) Win32k registered with CSRSS
fixme:(dll\win32\user32\win.c:3324) (hwnd 0000000C) stub!
Assertion 'TempPte.u.Soft.PageFileHigh == 0' failed at ARM³::PAGFAULT line 168
PuTTYEntered debugger on embedded INT3 at 0x0008:0x808dba8e.
kdb:> bt
Eip:
<ntoskrnl.exe:dba8f (lib\rtl\i386\debug_asm.S:33 (DbgBreakPoint@0))>
Frames:
<ntoskrnl.exe:8b194 (ARM³::PAGFAULT:168 (MiDispatchFault@32))>
<ntoskrnl.exe:8b64e (ARM³::PAGFAULT:372 (MmArmAccessFault@16))>
<ntoskrnl.exe:973db (ntoskrnl/mm/mmfault.c:286 (MmAccessFault@16))>
<ntoskrnl.exe:d4cce (ntoskrnl\ke\i386\trap.s:2003 (KiTrap14))>
<win32k.sys:2fe54 (subsystems/win32/win32k/dib/dib16bpp.c:424 (DIB_16BPP_ColorFill))>
<000000f0>
Couldn't access memory at 0x00FF53F4!
kdb:>