Fix crashes when rendering monitors of varying sizes

When calling .flip(), we limit the size of the buffer. However, this
limit is not reset when writing the next time, which means we get
out-of-bounds errors, even if the buffer is /technically/ big enough.

Clearing the buffer before drawing (rather than just resetting the
position) is enough to fix this.

Fixes #476 (and closes #477, which is a duplicate)

src/main/java/dan200/computercraft/client/render/TileEntityMonitorRenderer.java

@@ -170,7 +170,7 @@ public class TileEntityMonitorRenderer extends TileEntitySpecialRenderer<TileMon
                     }
 
                     ByteBuffer monitorBuffer = tboContents;
-                    monitorBuffer.position( 0 );
+                    monitorBuffer.clear();
                     for( int y = 0; y < height; y++ )
                     {
                         TextBuffer text = terminal.getLine( y ), textColour = terminal.getTextColourLine( y ), background = terminal.getBackgroundColourLine( y );