Fix several issues with char/paste event validation

- Fix isValidClipboard always returning true.
 - Fix characters >=128 being rejected. We changed the signature from a
   byte to an int in 0f123b5efd, but
   didn't update all call sites.

   Valhalla cannot come soon enough. I would love to be able to have
   (cheap) wrapper classes for some of these types.

See Zeus-guy's comments in #860.

projects/common/src/main/java/dan200/computercraft/shared/computer/menu/ServerInputState.java

@@ -74,7 +74,7 @@ public class ServerInputState<T extends AbstractContainerMenu & ComputerMenu> im
     }
 
     private static boolean isValidClipboard(ByteBuffer buffer) {
-        for (int i = buffer.remaining(), max = buffer.limit(); i < max; i++) {
+        for (int i = buffer.position(), max = buffer.limit(); i < max; i++) {
             if (!StringUtil.isTypableChar(buffer.get(i))) return false;
         }
         return true;

projects/core/src/main/java/dan200/computercraft/core/util/StringUtil.java

@@ -70,6 +70,17 @@ public final class StringUtil {
         };
     }
 
+    /**
+     * Check if a character is capable of being input and passed to a {@linkplain ComputerEvents#charTyped(ComputerEvents.Receiver, byte)
+     * "char" event}.
+     *
+     * @param chr The character to check.
+     * @return Whether this character can be typed.
+     */
+    public static boolean isTypableChar(byte chr) {
+        return isTypableChar(chr & 0xFF);
+    }
+
     /**
      * Check if a character is capable of being input and passed to a {@linkplain ComputerEvents#charTyped(ComputerEvents.Receiver, byte)
      * "char" event}.