[util] Implement parsing ratios from string views

src/util/util_ratio.h

 #include <numeric>
 #include <algorithm>
 #include <cstdint>
+#include <string>
+#include <charconv>
 
 namespace dxvk {
 
@@ -13,15 +15,39 @@ namespace dxvk {
   public:
 
     Ratio(T num, T denom) {
-      const T gcd = std::gcd(num, denom);
+      set(num, denom);
+    }
+
+    Ratio(std::string_view view) {
+      set(0, 0);
+
+      size_t colon = view.find(":");
+
+      if (colon == std::string_view::npos)
+        return;
+
+      std::string_view numStr   = view.substr(0, colon);
+      std::string_view denomStr = view.substr(colon + 1);
 
-      m_num    = num   / gcd;
-      m_denom  = denom / gcd;
+      T num = 0, denom = 0;
+      std::from_chars(numStr.data(),   numStr.data()   + numStr.size(),   num);
+      std::from_chars(denomStr.data(), denomStr.data() + denomStr.size(), denom);
+
+      set(num, denom);
     }
 
     inline T num()    const { return m_num; }
     inline T denom() const { return m_denom; }
 
+    inline bool undefined() const { return m_denom == 0; }
+
+    inline void set(T num, T denom) {
+      const T gcd = std::gcd(num, denom);
+
+      m_num = num / gcd;
+      m_denom = denom / gcd;
+    }
+
     inline bool operator == (const Ratio& other) const {
       return num() == other.num() && denom() == other.denom();
     }