Small canvas read/write fixes (#10955)

core/src/mindustry/logic/LExecutor.java

@@ -615,7 +615,7 @@ public class LExecutor{
                     toVar.isobj = value.isobj;
                 }
             }else if(from instanceof CanvasBuild canvas && (exec.privileged || (from.team == exec.team))){
-		        canvas.setPixel(address, (int)(value.numval));
+		        canvas.setPixel(address, value.numi());
             }
         }
     }

core/src/mindustry/world/blocks/logic/CanvasBlock.java

@@ -167,11 +167,11 @@ public class CanvasBlock extends Block{
             }
         }
 
-        public int getPixel(int pos){
+        public double getPixel(int pos){
             if(pos >= 0 && pos < canvasSize * canvasSize){
                 return getByte(data, pos * bitsPerPixel);
             }
-            return 0;
+            return Double.NaN;
         }
 
         public int getPixel(int x, int y){