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.cpp

@@ -77,6 +77,7 @@ NVIDIA HAS BEEN ADVISED OF THE POSSIBILITY OF SUCH DAMAGE.
 \****************************************************************************/
 
 #include <cstdlib>
+#include <locale>
 
 #include "PpContext.h"
 
@@ -91,6 +92,8 @@ TPpContext::TPpContext(TParseContextBase& pc, const std::string& rootFileName, T
     for (elsetracker = 0; elsetracker < maxIfNesting; elsetracker++)
         elseSeen[elsetracker] = false;
     elsetracker = 0;
+
+    strtodStream.imbue(std::locale::classic());
 }
 
 TPpContext::~TPpContext()