added txt.size() to return text screen width and height in 1 call. Also fixed txt.width/height on the C128 (where SCREEN reports 1 less for them...)

compiler/res/prog8lib/cx16/textio.p8

@@ -615,6 +615,13 @@ asmsub height() clobbers(X, Y) -> ubyte @A {
     }}
 }
 
+asmsub size() clobbers(A) -> ubyte @X, ubyte @Y {
+    ; -- returns the text screen width in X and height in Y (number of columns and rows)
+    %asm {{
+        jmp  cbm.SCREEN
+    }}
+}
+
 asmsub waitkey() -> ubyte @A {
     %asm {{
 -       jsr cbm.GETIN