[KERNEL] Add sysfn 39.3 - copy rect of background image to buffer.

Previously was only 39.2 that reads one pixel from background image.
(39.2 is shomehow useless due to big context switching overhead if you trying to read some area of pixels using 39.2)

git-svn-id: svn://kolibrios.org@9458 a494cfbc-eb01-0410-851d-a64ba20cac60

kernel/trunk/docs/sysfuncs.txt

@@ -2004,6 +2004,24 @@ Remarks:
   * There is a pair function to set pixel on the background image -
     subfunction 2 of function 15.
 
+---------------------- Constants for registers: ----------------------
+  eax - SF_BACKGROUND_GET_RECT (39)
+======================================================================
+== Function 39, subfunction 3 - get rect from the background image.  =
+======================================================================
+Parameters:
+  * eax = 39 - function number
+  * ebx = 3 - subfunction number
+  * ecx = [x]*65536 + [y]
+  * edx = [width]*65536 + [height]
+  * esi = buffer address
+Returned value:
+  * eax = 0 - success
+  * eax = -1 - error
+Remarks:
+  * Function copies specified rectangle of background image to buffer
+    in 0x00RRGGBB format
+
 ---------------------- Constants for registers: ----------------------
   eax - SF_BACKGROUND_GET (39)
 ======================================================================