Fix compile issues in the SVG plugin when using a non-std string type

[tretre91]

This PR fixes a few compile issues related to the use of std::string where a Rml::String is expected (or the opposite).

In SVGCache.cpp, I applied the same changes as in #820, since they got overwritten by #777. I also added an explanatory comment.

Also note that the samples and benchmarks don't compile when Rml::String is not std::string. I can add the fix for the samples in this PR if needed, I put the patch below.
For the benchmarks I don't know if it's necessary to fix them, as I wouldn't expect people to run them with a non-std string type. Afaict it's just that the bench.run(name, ...) function expects a std::string but is passed an Rml::String.

Patch for the samples

diff --git a/Samples/basic/svg/src/main.cpp b/Samples/basic/svg/src/main.cpp
index c0f5a797..7ad37699 100644
--- a/Samples/basic/svg/src/main.cpp
+++ b/Samples/basic/svg/src/main.cpp
@@ -115,7 +115,7 @@ int main(int /*argc*/, char** /*argv*/)
        Shell::LoadFonts();
 
        // Load and show the documents.
-       std::vector<std::string> rml_docs = {"basic/svg/data/svg_element.rml", "basic/svg/data/svg_decorator.rml", "basic/svg/data/svg_inline.rml"};
+       std::vector<Rml::String> rml_docs = {"basic/svg/data/svg_element.rml", "basic/svg/data/svg_decorator.rml", "basic/svg/data/svg_inline.rml"};
        for (const auto& rml_doc : rml_docs)
        {
                if (Rml::ElementDocument* document = context->LoadDocument(rml_doc))
diff --git a/Samples/basic/transform/src/main.cpp b/Samples/basic/transform/src/main.cpp
index e7eb7d7c..97445805 100644
--- a/Samples/basic/transform/src/main.cpp
+++ b/Samples/basic/transform/src/main.cpp
@@ -61,9 +61,7 @@ public:
 
                if (document && perspective > 0)
                {
-                       std::stringstream s;
-                       s << "perspective(" << perspective << "dp) ";
-                       document->SetProperty("transform", s.str());
+                       document->SetProperty("transform", Rml::CreateString("perspective(%fdp) ", perspective));
                }
        }
 
@@ -71,11 +69,11 @@ public:
        {
                if (document)
                {
-                       std::stringstream s;
+                       Rml::String s;
                        if (perspective > 0)
-                               s << "perspective(" << perspective << "dp) ";
-                       s << "rotate3d(0.0, 1.0, 0.0, " << degrees << "deg)";
-                       document->SetProperty("transform", s.str());
+                               s += Rml::CreateString("perspective(%fdp) ", perspective);
+                       s += Rml::CreateString("rotate3d(0.0, 1.0, 0.0, %fdeg)", degrees);
+                       document->SetProperty("transform", s);
                }
        }

[mikke89]

Thanks a lot! Yeah, we don't have any tests for this so I expect they will keep needing some maintenance. Thanks for making these ones as you notice them.

Feel free to make a separate PR for the samples as well. It's not the highest priority but I'd happily accept a PR.