Consider VRAM usage with NPOT textures

[Zylann]

I realized that the off-by-one required in terrain textures could become dreadful for VRAM usage, because it makes them non-power-of-two (NPOT). On graphics card, NPOT textures may be upscaled to fit the next power of two, and big terrains could end up occupying 4 times more memory to fit that 1-pixel padding...

Explanation about the 1-pixel padding: https://github.com/Zylann/godot_heightmap_module/blob/master/height_map_data.cpp#L22