GLX: note the implicit flushes with ReadPixels in indirect contexts.

This just notes the flush has occured when readpixels returns, and
fixes the glean test.

glx/singlepix.c

@@ -91,6 +91,7 @@ int __glXDisp_ReadPixels(__GLXclientState *cl, GLbyte *pc)
 	__GLX_SEND_HEADER();
 	__GLX_SEND_VOID_ARRAY(compsize);
     }
+    __GLX_NOTE_FLUSHED_CMDS(cx);
     return Success;
 }
 

glx/singlepixswap.c

@@ -102,6 +102,7 @@ int __glXDispSwap_ReadPixels(__GLXclientState *cl, GLbyte *pc)
 	__GLX_SEND_HEADER();
 	__GLX_SEND_VOID_ARRAY(compsize);
     }
+    __GLX_NOTE_FLUSHED_CMDS(cx);
     return Success;
 }