Context: use glGetString(GL_EXTENSIONS) on GL 2.1.

glGetStringi() is since GL 3.0.

src/Context.cpp

@@ -165,6 +165,7 @@ Context::Context() {
             futureExtensions.insert(make_pair(extension._string, extension));
 
     /* Check for presence of extensions in future versions */
+    if(isVersionSupported(Version::GL300)) {
         GLuint index = 0;
         const char* extension;
         while((extension = reinterpret_cast<const char*>(glGetStringi(GL_EXTENSIONS, index++)))) {
@@ -175,6 +176,18 @@ Context::Context() {
             }
         }
 
+    /* OpenGL 2.1 doesn't have glGetStringi() */
+    } else {
+        vector<string> extensions = Corrade::Utility::split(reinterpret_cast<const char*>(glGetString(GL_EXTENSIONS)), ' ');
+        for(const string& extension: extensions) {
+            auto found = futureExtensions.find(extension);
+            if(found != futureExtensions.end()) {
+                _supportedExtensions.push_back(found->second);
+                extensionStatus.set(found->second._index);
+            }
+        }
+    }
+
     /* Set this context as current */
     CORRADE_ASSERT(!_current, "Context: Another context currently active", );
     _current = this;