Use per-screen Pixmaps for glyphs

Instead of system-memory data which prevents accelerated
compositing of glyphs, (at least without forcing an upload
of the glyph data before compositing).

render/glyphstr.h

@@ -44,9 +44,11 @@ typedef struct _Glyph {
     unsigned char   sha1[20];
     CARD32	    size; /* info + bitmap */
     xGlyphInfo	    info;
-    /* bits follow */
+    /* per-screen pixmaps follow */
 } GlyphRec, *GlyphPtr;
 
+#define GlyphPixmap(glyph) ((PixmapPtr *) ((glyph) + 1))
+
 typedef struct _GlyphRef {
     CARD32	signature;
     GlyphPtr	glyph;