refs #60: first cut of POI rendering

* add basic marker style to stylesheet
 * pass through marker style to PBF conversion
 * marker/point tile parsing, bucket, buffer, shaders, etc.
 * new sprite hosting for 1x/2x variants
 * sprite load accepts screen scale argument

src/geometry/point_buffer.cpp

@@ -0,0 +1,12 @@
+#include <llmr/geometry/point_buffer.hpp>
+#include <llmr/platform/gl.hpp>
+
+#include <cmath>
+
+using namespace llmr;
+
+void PointVertexBuffer::add(vertex_type x, vertex_type y) {
+    vertex_type *vertices = static_cast<vertex_type *>(addElement());
+    vertices[0] = x;
+    vertices[1] = y;
+}