glamor: Add support for a1 composite sources.

They're stored just like a8, but the values are set to either 0.0 or
1.0.  Because they're a8 with only two legal values, we can't use them
as destinations, but nobody's rendering to a1 dests anyway (we hope).

glamor/glamor_render.c

@@ -456,6 +456,7 @@ static Bool
 good_source_format(PicturePtr picture)
 {
     switch (picture->format) {
+    case PICT_a1:
     case PICT_a8:
     case PICT_a8r8g8b8:
 	return TRUE;
@@ -477,6 +478,7 @@ static Bool
 good_mask_format(PicturePtr picture)
 {
     switch (picture->format) {
+    case PICT_a1:
     case PICT_a8:
     case PICT_a8r8g8b8:
 	return TRUE;