Rewrite glop texture asserts

bug:19641517

Also switch Glop VertexAttribFlags to use int for group of flags.

Change-Id: Ib7b1934197a62206a55baa6ab484ac59f5bec816
diff --git a/libs/hwui/Glop.h b/libs/hwui/Glop.h
index 62da6e08..2c6f6c1 100644
--- a/libs/hwui/Glop.h
+++ b/libs/hwui/Glop.h
@@ -78,7 +78,7 @@
         // TODO: enforce mutual exclusion with restricted setters and/or unions
         struct Vertices {
             GLuint bufferObject;
-            VertexAttribFlags flags;
+            int attribFlags;
             const void* position;
             const void* texCoord;
             const void* color;