[core] Split layer type specific code in mbgl::Programs

Progams code for a certain layer type is encapsulted within
a dedicated `<layer type>Programs` class, inherited from
the generic base `LayerTypePrograms` class.

`mbgl::Programs::get<layer type>Programs()` lazily initializes the
layer type-specific programs code using pointer to the base class,
which allows LTO to remove this code from binaries (if the corresponding
`get<layer type>Programs()` method can never be invoked).

src/mbgl/programs/programs.cpp

@@ -0,0 +1,87 @@
+#include <mbgl/programs/programs.hpp>
+
+#include <mbgl/programs/background_program.hpp>
+#include <mbgl/programs/circle_program.hpp>
+#include <mbgl/programs/heatmap_program.hpp>
+#include <mbgl/programs/hillshade_program.hpp>
+#include <mbgl/programs/fill_extrusion_program.hpp>
+#include <mbgl/programs/fill_program.hpp>
+#include <mbgl/programs/line_program.hpp>
+#include <mbgl/programs/raster_program.hpp>
+#include <mbgl/programs/symbol_program.hpp>
+
+namespace mbgl {
+
+Programs::Programs(gl::Context& context_, const ProgramParameters& programParameters_)
+    : debug(context_, programParameters_),
+      clippingMask(context_, programParameters_),
+      context(context_),
+      programParameters(std::move(programParameters_)) {
+}
+
+Programs::~Programs() = default;
+
+BackgroundLayerPrograms& Programs::getBackgroundLayerPrograms() noexcept {
+    if (!backgroundPrograms) {
+        backgroundPrograms = std::make_unique<BackgroundLayerPrograms>(context, programParameters);
+    }
+    return static_cast<BackgroundLayerPrograms&>(*backgroundPrograms);   
+}
+
+RasterLayerPrograms& Programs::getRasterLayerPrograms() noexcept {
+    if (!rasterPrograms) {
+        rasterPrograms = std::make_unique<RasterLayerPrograms>(context, programParameters);
+    }
+    return static_cast<RasterLayerPrograms&>(*rasterPrograms);   
+}
+
+HeatmapLayerPrograms& Programs::getHeatmapLayerPrograms() noexcept {
+    if (!heatmapPrograms) {
+        heatmapPrograms = std::make_unique<HeatmapLayerPrograms>(context, programParameters);
+    }
+    return static_cast<HeatmapLayerPrograms&>(*heatmapPrograms);   
+}
+
+HillshadeLayerPrograms& Programs::getHillshadeLayerPrograms() noexcept {
+    if (!hillshadePrograms) {
+        hillshadePrograms = std::make_unique<HillshadeLayerPrograms>(context, programParameters);
+    }
+    return static_cast<HillshadeLayerPrograms&>(*hillshadePrograms);   
+}
+
+FillLayerPrograms& Programs::getFillLayerPrograms() noexcept {
+    if (!fillPrograms) {
+        fillPrograms = std::make_unique<FillLayerPrograms>(context, programParameters);
+    }
+    return static_cast<FillLayerPrograms&>(*fillPrograms);   
+}
+
+FillExtrusionLayerPrograms& Programs::getFillExtrusionLayerPrograms() noexcept {
+    if (!fillExtrusionPrograms) {
+        fillExtrusionPrograms = std::make_unique<FillExtrusionLayerPrograms>(context, programParameters);
+    }
+    return static_cast<FillExtrusionLayerPrograms&>(*fillExtrusionPrograms);   
+}
+
+CircleLayerPrograms& Programs::getCircleLayerPrograms() noexcept {
+    if (!circlePrograms) {
+        circlePrograms = std::make_unique<CircleLayerPrograms>(context, programParameters);
+    }
+    return static_cast<CircleLayerPrograms&>(*circlePrograms);   
+}
+
+LineLayerPrograms& Programs::getLineLayerPrograms() noexcept {
+    if (!linePrograms) {
+        linePrograms = std::make_unique<LineLayerPrograms>(context, programParameters);
+    }
+    return static_cast<LineLayerPrograms&>(*linePrograms);   
+}
+
+SymbolLayerPrograms& Programs::getSymbolLayerPrograms() noexcept {
+    if (!symbolPrograms) {
+        symbolPrograms = std::make_unique<SymbolLayerPrograms>(context, programParameters);
+    }
+    return static_cast<SymbolLayerPrograms&>(*symbolPrograms);   
+}
+
+} // namespace mbgl