Image tiling performance improvements and device support

[oscarlorentzon]

The current implementation of image tiling is creating a texture with the size of the original image. This is memory inefficient for large images, e.g. panoramas with sizes up to 16384 x 8192 px, and causes long rendering times leaving the viewer unresponsive for up to hundreds of milliseconds from time to time. On devices where the WebGL max texture size is smaller than the original image size the texture will just be scaled and it will not be possible to view the full resolution even when zooming.

The idea with this issue is to only create a texture with the amount of pixels that is needed for the current viewport size. That reduces the memory requirements, the WebGL max texture size will not be a limiting factor, the rendering speed will be improved, and the viewer should behave more smoothly when panning/tilting/zooming.