exa: A simple 3rd backend implementation.

- Based on driver pixmaps with some changes (completely transparent to driver).
- It helps with the problem of known software fallbacks, such as trapezoids.
- exaDoMigration is now called for all cases that provide a do_migration hook.
- exa_migration.c is renamed to exa_migration_classic.c

exa/exa_glyphs.c

@@ -385,10 +385,10 @@ exaGlyphCacheUploadGlyph(ScreenPtr         pScreen,
     if (pGlyphPixmap->drawable.bitsPerPixel != pCachePixmap->drawable.bitsPerPixel)
 	goto composite;
 
-    /* cache pixmap must be offscreen. */
-    if (pExaPixmap->pDamage) {
+    if (pExaScr->do_migration) {
 	ExaMigrationRec pixmaps[1];
 
+	/* cache pixmap must be offscreen. */
 	pixmaps[0].as_dst = TRUE;
 	pixmaps[0].as_src = FALSE;
 	pixmaps[0].pPix = pCachePixmap;