Reduce the memory usage of elevation textures

ℹ️ Funded by an industrial customer.

Currently, elevation textures are 4-channel (RGBAFormat) and 4 bytes per channel (FloatType). This uses 16 bytes per pixel.

However, elevation data only requires one channel for the data, and one channel for the transparency/no-data information.

We can cut memory usage of elevation textures by 50% this way.