Add shader registry (#924)

src/mbgl/programs/program_parameters.cpp

@@ -1,5 +1,6 @@
 #include <mbgl/programs/program_parameters.hpp>
 #include <mbgl/util/string.hpp>
+#include <string_view>
 
 namespace mbgl {
 
@@ -15,11 +16,28 @@ ProgramParameters::ProgramParameters(const float pixelRatio,
               result += "#define OVERDRAW_INSPECTOR\n";
           }
           return result;
-      }()) {
+      }())
+{}
+
+ProgramParameters ProgramParameters::withShaderSource(std::string_view vertexSource,
+    std::string_view fragmentSource) const noexcept
+{
+    ProgramParameters params = *this;
+    params.vertexSource_ = vertexSource;
+    params.fragmentSource_ = fragmentSource;
+    return params;
 }
 
 const std::string& ProgramParameters::getDefines() const {
     return defines;
 }
 
+const std::string& ProgramParameters::vertexSource() const noexcept {
+    return vertexSource_;
+}
+
+const std::string& ProgramParameters::fragmentSource() const noexcept {
+    return fragmentSource_;
+}
+
 } // namespace mbgl