Port scanline algorithm from _getCoveringTiles()

fixes #22

include/llmr/map/coverage.hpp

@@ -0,0 +1,13 @@
+#ifndef LLMR_MAP_COVERAGE
+#define LLMR_MAP_COVERAGE
+
+#include <llmr/util/vec.hpp>
+#include <forward_list>
+
+namespace llmr {
+
+std::forward_list<vec3<int32_t>> covering_tiles(int32_t zoom, const box& points);
+
+}
+
+#endif

include/llmr/map/transform.hpp

@@ -51,7 +51,7 @@ public:
     void stopScaling();
 
     // Temporary
-    void mapCornersToBox(uint32_t z, box& b) const;
+    box mapCornersToBox(uint32_t z) const;
 
 private:
     void setScaleXY(double new_scale, double xn, double yn, double duration = 0);

include/llmr/util/vec.hpp

@@ -55,6 +55,7 @@ struct vec4 {
 
 struct box {
     vec2<double> tl, tr, bl, br;
+    vec2<double> center;
 };
 
 }