Establish S52 PLIB default colour scheme.
Merge branch 'master' of https://github.com/OpenCPN/OpenCPN into optimum-merging
Revert "look up correct color scheme index if not set"

This reverts commit 368917ff6a5d4217b93b8f5ed777a8ad2943ed68.
major rework of framebuffer object and raster chart rendering

These changes greatly improve performance for
- idle cpu (cached texture works in all modes even rotated) if the viewport didn't change
- opengles (embedded systems) use different formats for texture rectangles
- overzoom uses floating point coordinates for sub-pixes alignment to avoid wobbling
- reduced video memory use, only needed mipmaps are loaded
- many items (routes, waypoints tides etc) can take advantage of framebuffer object
- memory use is based on bytes rather than textures because of different sizes for mipmaps
- previous version with basemult caused a lot of memory traffic to gpu
avoid crashing on start if opengl extensions cannot be loaded
texture compression caching support

use software libraries to generate compressed textures for speed/quality reasons
   and to support etc1 compression for opengles implementations
add lz4 library to compress compressed textures for optimal disk storage
this support greatly reduces video and ram usage
raster charts load about 20 times faster
allow zooming out a bit farther for world background chart
fix canvas render region calculation for non-quilted cm93 charts in opengl mode

I really don't understand exactly why this is needed, but it completely
fixes the issue for non-quilted rotated vector charts if the border of a cell is nearby
I suspect there is a much more elegant fix