[core] reparse tiles when new data arrives

We're now reparsing tiles when they expire. We're also swapping out buckets atomically to avoid flickering data; i.e. we're displaying the old data as long as we don't have a new parsed bucket for that layer yet. The parsed buckets now live in the *TileData objects rather than in the TileWorker; only partially parsed == pending buckets will remain in the TileWorker. Once they're parsed, they're moved to the *TileData object.

src/mbgl/renderer/debug_bucket.cpp

@@ -5,21 +5,13 @@
 #include <mbgl/platform/gl.hpp>
 
 #include <cassert>
+#include <string>
 
 using namespace mbgl;
 
-DebugBucket::DebugBucket(DebugFontBuffer& fontBuffer_)
-    : fontBuffer(fontBuffer_) {
-}
-
-void DebugBucket::upload() {
-    fontBuffer.upload();
-
-    uploaded = true;
-}
-
-void DebugBucket::render(Painter& painter, const StyleLayer&, const TileID&, const mat4& matrix) {
-    painter.renderDebugText(*this, matrix);
+DebugBucket::DebugBucket(const TileID id, const TileData::State state_) : state(state_) {
+    const std::string text = std::string(id) + " - " + TileData::StateToString(state);
+    fontBuffer.addText(text.c_str(), 50, 200, 5);
 }
 
 void DebugBucket::drawLines(PlainShader& shader) {