Add some wrapper for resources (e.g. pixmaps) that frees them on drop (free_pixmap())

[psychon]

I end up writing something like the following in most of the examples:

x11rb/examples/xeyes.rs

Lines 151 to 156 in 52b21fa

	struct FreePixmap<'c, C: Connection>(&'c C, PIXMAP);
	impl<C: Connection> Drop for FreePixmap<'_, C> {
	fn drop(&mut self) {
	self.0.free_pixmap(self.1).unwrap();
	}
	}

A proper implementation would add Deref, I guess. And some wrappers around things like CreatePixmap, although that is harder to do automatically...

[psychon]

I gave this a go and I am not too happy about the necessary amount of code, not about the unwrap(). Perhaps this can be simplified with a macro? How "easily usable" would the result be?

// CreateWindow/DestroyWindow
// OpenFont/CloseFont
// CreatePixmap/FreePixmap
// CreateGC/FreeGC
// CreateColormap/FreeColormap
// CreateCursor/CreateGlyphCursor/FreeCursor
//
// Plus the same for all the extensions that are out there.

/// TODO
#[derive(Debug)]
pub struct FreePixmap<'c, C: Connection>(&'c C, u32);

impl<'c, C: Connection> FreePixmap<'c, C>
{
    /// TODO
    pub fn for_pixmap(conn: &'c C, pixmap: PIXMAP) -> Self
    {
        FreePixmap(conn, pixmap)
    }

    /// TODO
    pub fn create_pixmap(conn: &'c C, depth: u8, drawable: u32, width: u16, height: u16) -> Result<Self, ConnectionError> {
        let pixmap = conn.generate_id();
        drop(conn.create_pixmap(depth, pixmap, drawable, width, height)?);
        Ok(Self::for_pixmap(conn, pixmap))
    }

    pub fn pixmap(&self) -> PIXMAP {
        self.1
    }
}

impl<C: Connection> Deref for FreePixmap<'_, C>
{
    type Target = PIXMAP;

    fn deref(&self) -> &PIXMAP {
        &self.1
    }
}

impl<C: Connection> Drop for FreePixmap<'_, C>
{
    fn drop(&mut self) {
        drop(self.0.free_pixmap(self.1).unwrap());
    }
}

[psychon]

#620 implemented the xproto part of this. For this issue, the remaining pieces are:

• Also add wrapper objects for all the various extension objects.
• Instead of hand-writing all of this code, integrate this into the code generator.

I think the second bullet point should be addressed before the first one. Hand-writing this stuff for xproto was already annoying enough. :-)