Add basic Exa driver entry point validation.

In exaDriverInit(), quickly check if the programmer forgot to set some
mandadory driver hooks, in that case display a meaningful error message.

exa/exa.c

@@ -722,6 +722,48 @@ exaDriverAlloc(void)
     return xcalloc(1, sizeof(ExaDriverRec));
 }
 
+static Bool
+exaDriverValidateEntryPoints (ExaDriverPtr  pExaDriver)
+{
+    Bool res=TRUE ;
+
+    if (!pExaDriver)
+        return FALSE ;
+
+    if (!pExaDriver->memoryBase) {
+        LogMessage(X_ERROR,
+                   "Exa: Exa::memoryBase member "
+                   "must be assigned to a value different from zero\n") ;
+        res = FALSE ;
+    }
+    if (!pExaDriver->memorySize) {
+        LogMessage(X_ERROR,
+                   "Exa: Exa::memorySize member must be different from zero\n") ;
+        res = FALSE ;
+    }
+    if (pExaDriver->offScreenBase > pExaDriver->memorySize) {
+        LogMessage(X_ERROR,
+                   "Exa: Exa::ffscreenBase must be <= pExaDriver->memorySize member\n") ;
+        res = FALSE ;
+    }
+    if (!pExaDriver->PrepareSolid) {
+        LogMessage(X_ERROR,
+                   "Exa: Exa::PrepareSolid member is required to be non NULL\n") ;
+        res = FALSE ;
+    }
+    if (!pExaDriver->PrepareCopy) {
+        LogMessage(X_ERROR,
+                   "Exa: Exa::PrepareCopy member is required to be non NULL\n") ;
+        res = FALSE ;
+    }
+    if (!pExaDriver->WaitMarker) {
+        LogMessage(X_ERROR,
+                   "Exa: Exa::WaitWarker member is required to be non NULL\n") ;
+        res = FALSE ;
+    }
+    return res ;
+}
+
 /**
  * @param pScreen screen being initialized
  * @param pScreenInfo EXA driver record
@@ -806,6 +848,12 @@ exaDriverInit (ScreenPtr		pScreen,
     {
         pScreenInfo->maxPitchPixels = pScreenInfo->maxX;
     }
+    if (!exaDriverValidateEntryPoints(pScreenInfo))
+    {
+	LogMessage(X_ERROR, "Exa(%d): EXA driver entry points validation failed\n",
+		   pScreen->myNum) ;
+	return FALSE ;
+    }
 
 #ifdef RENDER
     ps = GetPictureScreenIfSet(pScreen);