Aerial image brightness adjust

这个补丁用于修正一些谷歌的卫星地图的亮度。在谷歌地图上，有些地方是有倾斜摄影的3D的模型的，这些地图的底图的亮度比其他地方都要高，如果在这些地方上空飞行的话，就会觉得颜色非常的不爽。

我用了一个多元方程将一些地方的亮度调低。但会带来一些小的副作用，主要是在一些亮度比较高的雪山，如果一些地方有石头，则会显得比较暗淡。如果从远处看的话，倒是没有太大的影响。

Google aerial has images for photogrammetry has brighter images than other areas, this is not so realistic when fly above these areas. You can see the borders of photogrammetry areas (brighter) and non photogrammetry areas (default darker color)

This is to somehow fix the issue by decrease too bright areas. This is done using an function by calculating the brightness. Some side effort you can find out is in some mountain areas, which covered by snow, some areas has rocks may darker. If you see it far away, it is not obvious.