[util] Expose frame rate environment variable

We'll need this in more places.

src/util/util_fps_limiter.cpp

@@ -13,15 +13,11 @@ using namespace std::chrono_literals;
 namespace dxvk {
   
   FpsLimiter::FpsLimiter() {
-    std::string env = env::getEnvVar("DXVK_FRAME_RATE");
+    auto override = getEnvironmentOverride();
 
-    if (!env.empty()) {
-      try {
-        setTargetFrameRate(std::stod(env), 0);
-        m_envOverride = true;
-      } catch (const std::invalid_argument&) {
-        // no-op
-      }
+    if (override) {
+      setTargetFrameRate(*override, 0);
+      m_envOverride = true;
     }
   }
 
@@ -128,4 +124,19 @@ namespace dxvk {
     return false;
   }
 
+
+  std::optional<double> FpsLimiter::getEnvironmentOverride() {
+    std::string env = env::getEnvVar("DXVK_FRAME_RATE");
+
+    if (!env.empty()) {
+      try {
+        return std::stod(env);
+      } catch (const std::invalid_argument&) {
+        // no op
+      }
+    }
+
+    return std::nullopt;
+  }
+
 }

src/util/util_fps_limiter.h

@@ -1,6 +1,7 @@
 #pragma once
 
 #include <array>
+#include <optional>
 
 #include "thread.h"
 #include "util_time.h"
@@ -39,6 +40,12 @@ namespace dxvk {
      */
     void delay();
 
+    /**
+     * \brief Queries environment override
+     * \returns Frame rate given by environment override
+     */
+    static std::optional<double> getEnvironmentOverride();
+
   private:
 
     using TimePoint = dxvk::high_resolution_clock::time_point;