PP: Implement locale-independent strtod, using istringstream and a fast path.

Fixes #1228. Fixes #234.

This uses imbue() to be locale independent.  Notes:

- 'sstream >> double' is much slower than strtod()
  * this was measurable in the test suite as a whole, despite being
    a tiny fraction of what the test suite does
- so, this embeds a fast path that bypasses sstream most of the time
  => the test suite is faster than before
- sstream is probably slower, because it does more accurate rounding than strtod()
- sstream does not create INFINITY by itself, this was done based on failure inferencing

glslang/MachineIndependent/preprocessor/PpContext.h

@@ -80,6 +80,7 @@ NVIDIA HAS BEEN ADVISED OF THE POSSIBILITY OF SUCH DAMAGE.
 
 #include <stack>
 #include <unordered_map>
+#include <sstream>
 
 #include "../ParseHelper.h"
 
@@ -620,6 +621,8 @@ protected:
     std::string rootFileName;
     std::stack<TShader::Includer::IncludeResult*> includeStack;
     std::string currentSourceFile;
+
+    std::istringstream strtodStream;
 };
 
 } // end namespace glslang