Heatmap layer (#1230)

Co-authored-by: pre-commit-ci[bot] <66853113+pre-commit-ci[bot]@users.noreply.github.com>

src/mbgl/renderer/renderer_impl.cpp

@@ -21,6 +21,7 @@
 #include <mbgl/gfx/drawable_tweaker.hpp>
 #include <mbgl/gl/drawable_gl.hpp>
 #include <mbgl/renderer/layer_tweaker.hpp>
+#include <mbgl/renderer/render_target.hpp>
 #include <mbgl/shaders/gl/shader_program_gl.hpp>
 #endif
 
@@ -142,6 +143,9 @@ void Renderer::Impl::render(const RenderTree& renderTree) {
 #if MLN_DRAWABLE_RENDERER
         // Give the layers a chance to upload
         orchestrator.observeLayerGroups([&](LayerGroupBase& layerGroup) { layerGroup.upload(*uploadPass); });
+
+        // Give the render targets a chance to upload
+        orchestrator.observeRenderTargets([&](RenderTarget& renderTarget) { renderTarget.upload(*uploadPass); });
 #endif
     }
 
@@ -176,6 +180,12 @@ void Renderer::Impl::render(const RenderTree& renderTree) {
 #endif
     }
 
+#if MLN_DRAWABLE_RENDERER
+    // draw render targets
+    orchestrator.observeRenderTargets(
+        [&](RenderTarget& renderTarget) { renderTarget.render(orchestrator, renderTree, parameters); });
+#endif
+
     // - CLEAR
     // -------------------------------------------------------------------------------------
     // Renders the backdrop of the OpenGL view. This also paints in areas where