Fix STB library conflicts

[csciguy8]

Community contribution to fix an issue where cesium-native's use of the STB library can conflict with an application's use of it.

STB has more than one implementation that can be defined:

• The community contribution solved a problem with STB_IMAGE_IMPLEMENTATION.
• We solved a similar problem with STB_IMAGE_RESIZE_IMPLEMENTATION in Move stb resize function to Cesium namespace #871.
• I additionally rounded this out by making STB_IMAGE_WRITE_IMPLEMENTATION static as well to avoid any similar issues in the future.

[kring]

Thanks @csciguy8!