Improving sand patterns

[kocio-pl]

Related to #1990 and #2746.

1. The main problem is that sand pattern (beach.png) is invisible for me at 100%, it just makes the natural=beach/sand area darker. The pattern become apparent at about 200%. However the pattern beach_coarse.png (used for gravel et co., but also for reef) looks already this way.

There may be few ways to correct it. We could just replace beach.png with current beach_coarse.png and then make one or more of the following:

• enlarge also beach_coarse.png
• use bigger elements for beach_coarse.png
• use darker color for beach_coarse.png

2. There's also smaller naming problem - beach.png should be rather more universal sand.png (as it's used for sand areas, but not for all the beach types) and beach_coarse.png might be renamed to a more universal name too (like coarse.png or gravel.png).

3. Documentation could be improved by adding original options/links from pattern generator, if possible.

[jeisenbe]

@kocio-pl do you still think that the beach.png pattern is not clearly visible, except as making the surface appear darker? This would suggest removing it from natural=sand since sand has it's own fill color which can be adjusted darker or lighter.

(Note: I do not have a problem seeing the difference between beach without the pattern, beach.png and beach_coarse.png, they look like different patterns on the same color to me)

[kocio-pl]

I think this is way too fast to go into details of a single feature rendering when we're discussing more basic stuff.

You've already opened two sand-related PRs and just yesterday some structured proposition has appeared, that gives hope to make some clarity in all the rock/sand/beach mix - #3957 (comment) - which you have already commented 3 times (!) and we're waiting for a response to that... Adding even more discussion where more clarity is needed is counterproductive.

Please, hold on with that until we settle a system for what should be rendered with a color and/or overlay.