Faster and more correct tile fraction calculation

[mourner]

Makes the tile fraction calculation more correct (IMO) since it's now a direct projection rather than a combination of projection and linear interpolation inside a tile. It's also much much faster because it doesn't do unnecessary work.

Interestingly, this also addresses the discrepancy between Node 0.10 and 0.12 described here mapbox/tile-cover#63 (comment) — after this fix, tile-cover will behave in 0.10 exactly like in 0.12.

cc @morganherlocker

[mourner]

It's so much faster that it makes tile-cover 1.5-2 times faster as a result:

# with this tilebelt patch
scan point - z6 - z6 x 6,650,813 ops/sec ±2.49% (23 runs sampled)
scan road - z6 - z6 x 239,385 ops/sec ±3.63% (22 runs sampled)
scan road - z8 - z8 x 234,069 ops/sec ±4.15% (21 runs sampled)
scan road - z10 - z10 x 231,450 ops/sec ±4.37% (20 runs sampled)
scan road - z12 - z12 x 239,066 ops/sec ±3.86% (21 runs sampled)
scan road - z18 - z18 x 35,166 ops/sec ±2.80% (22 runs sampled)
scan road - z20 - z20 x 15,179 ops/sec ±1.83% (22 runs sampled)
scan road - z25 - z25 x 192 ops/sec ±2.38% (20 runs sampled)
scan road - z28 - z28 x 40.59 ops/sec ±4.34% (15 runs sampled)
scan building - z6 - z6 x 95,254 ops/sec ±1.64% (22 runs sampled)
scan building - z8 - z8 x 90,872 ops/sec ±3.29% (21 runs sampled)
scan building - z10 - z10 x 93,993 ops/sec ±1.46% (23 runs sampled)
scan building - z12 - z12 x 94,611 ops/sec ±2.98% (22 runs sampled)
scan building - z18 - z18 x 49,377 ops/sec ±1.71% (20 runs sampled)
scan building - z20 - z20 x 17,095 ops/sec ±6.78% (21 runs sampled)
scan building - z25 - z25 x 54.95 ops/sec ±3.22% (16 runs sampled)
scan building - z28 - z28 x 4.60 ops/sec ±8.03% (7 runs sampled)
scan russia polygon - z6 - z6 x 2,270 ops/sec ±3.06% (20 runs sampled)
scan russia polygon - z8 - z8 x 301 ops/sec ±2.19% (21 runs sampled)
scan russia polygon - z10 - z10 x 23.05 ops/sec ±6.52% (12 runs sampled)
scan russia polyline - z6 - z6 x 6,711 ops/sec ±2.03% (22 runs sampled)
scan russia polyline - z8 - z8 x 2,381 ops/sec ±2.31% (22 runs sampled)
scan russia polyline - z10 - z10 x 634 ops/sec ±2.43% (22 runs sampled)
scan russia polygon multizoom - z0 - z9 x 58.56 ops/sec ±3.61% (16 runs sampled)

# without the patch
scan point - z6 - z6 x 4,798,890 ops/sec ±1.50% (23 runs sampled)
scan road - z6 - z6 x 149,938 ops/sec ±2.87% (20 runs sampled)
scan road - z8 - z8 x 148,930 ops/sec ±3.35% (23 runs sampled)
scan road - z10 - z10 x 153,403 ops/sec ±2.92% (22 runs sampled)
scan road - z12 - z12 x 153,565 ops/sec ±3.08% (19 runs sampled)
scan road - z18 - z18 x 31,702 ops/sec ±3.01% (22 runs sampled)
scan road - z20 - z20 x 16,745 ops/sec ±1.55% (24 runs sampled)
scan road - z25 - z25 x 198 ops/sec ±2.50% (21 runs sampled)
scan road - z28 - z28 x 41.28 ops/sec ±5.27% (15 runs sampled)
scan building - z6 - z6 x 57,995 ops/sec ±2.36% (23 runs sampled)
scan building - z8 - z8 x 59,613 ops/sec ±1.83% (20 runs sampled)
scan building - z10 - z10 x 59,473 ops/sec ±1.61% (23 runs sampled)
scan building - z12 - z12 x 59,990 ops/sec ±3.07% (22 runs sampled)
scan building - z18 - z18 x 36,311 ops/sec ±1.33% (21 runs sampled)
scan building - z20 - z20 x 15,687 ops/sec ±4.06% (24 runs sampled)
scan building - z25 - z25 x 60.63 ops/sec ±2.79% (17 runs sampled)
scan building - z28 - z28 x 5.16 ops/sec ±3.96% (7 runs sampled)
scan russia polygon - z6 - z6 x 1,969 ops/sec ±1.77% (23 runs sampled)
scan russia polygon - z8 - z8 x 319 ops/sec ±2.45% (22 runs sampled)
scan russia polygon - z10 - z10 x 23.30 ops/sec ±5.62% (12 runs sampled)
scan russia polyline - z6 - z6 x 3,757 ops/sec ±1.38% (22 runs sampled)
scan russia polyline - z8 - z8 x 1,835 ops/sec ±2.13% (22 runs sampled)
scan russia polyline - z10 - z10 x 610 ops/sec ±2.20% (22 runs sampled)
scan russia polygon multizoom - z0 - z9 x 58.39 ops/sec ±3.38% (16 runs sampled)