vwebp: clear canvas during resize w/o animation

this corrects the checkboard pattern displayed with transparent images

Change-Id: I5f46dbc9fa3893d61f5f1d4fda643ac030238f94
(cherry picked from commit 67c25ad5b4)

examples/vwebp.c

@@ -264,6 +264,7 @@ static void HandleReshape(int width, int height) {
   glLoadIdentity();
   kParams.viewport_width = width;
   kParams.viewport_height = height;
+  if (!kParams.has_animation) ClearPreviousFrame();
 }
 
 static void PrintString(const char* const text) {