Great improvement regarding issue #22

- index.html now sets L_PREFER_CANVAS before loading Leaflet.js  This makes Leaflet use the Canvas instead of SVG, for generally better vide performance - and implicitly makes the trails easier to tap without using that “trail buffers” trick

- Removed the invisible trails which were intended to make tapping easier. This means OVERLAYS['trailbuffers'] and TRAILS_STYLE_INVISIBLE_BUFFER are gone, and the is_old_iphone() function has been removed as well

- Unrelated but opportunity: FISHINGAREAS_STYLE no longer specifies smoothFactor and lets it default to 1. Made no visible difference, and docs state that this should improve video performance.