Web GL Mipmapping and Linear Smoothing

I added some compiler flags for those who want Mipmapping or Linear smoothing enabled in WebGL. 

For Mipmapping, use: -D flambe_webgl_enable_mipmapping

For Linear smoothing use: -D flambe_webgl_enable_linear

Remember that mipmapping adds 33% more memory for all the extra textures that are getting generated. That's why I added the Linear flag in case you wanted smoothing graphics without using more memory. But keep in mind Linear smoothing will require heavier processing.