Use MIN(A,B) macro as defined in Utils.h. Add MAX(A,B) macro in Utils.h,
 as well. Don't have another set of MIN/MAX macro definitions in
 Screen.c.

 Amends the following compiler warnings:

 ```
 Screen.c:3600:0: warning: "MIN" redefined
  #define MIN(a,b) ((a) < (b)) ? (a) : (b);
  ^
 In file included from Screen.c:78:0:
 Utils.h:36:0: note: this is the location of the previous definition
  #define MIN(A, B) ( (A) < (B) ? (A) : (B) )
  ^
 ```