OSMesa: Use framebuffer size for framebuffer

src/osmesa_context.c

@@ -37,7 +37,7 @@ static void makeContextCurrentOSMesa(_GLFWwindow* window)
     if (window)
     {
         int width, height;
-        _glfwPlatformGetWindowSize(window, &width, &height);
+        _glfwPlatformGetFramebufferSize(window, &width, &height);
 
         // Check to see if we need to allocate a new buffer
         if ((window->context.osmesa.buffer == NULL) ||