Unable to read float32 texture on Chrome Android

[kjerandp]

Recent change #119 broke surfaces in mobile browsers. This is likely due to missing support for reading float32 textures. Float textures should be supported in most mobile browsers, but I suspect it is limited to 16 bit floating points.

We need to use a fallback where 32 bit floating points are not supported. Previously, we packed the data as 4x unsigned byte data, but only used the RGB channels. We should fallback to this approach where 32 bit floats are not supported, but then use all 4 channels for better precision. We can also perhaps use half-floats (16 bits) - as I would assume this will be used internally in the shader with these devices.