GL: use public API of the Extension class.

No need to friend and use underscored members.

src/Magnum/Context.cpp

@@ -646,7 +646,7 @@ bool Context::tryCreate() {
     /* Mark all extensions from past versions as supported */
     for(std::size_t i = 0; i != future; ++i)
         for(const Extension& extension: Extension::extensions(versions[i]))
-            _extensionStatus.set(extension._index);
+            _extensionStatus.set(extension.index());
 
     /* List of extensions from future versions (extensions from current and
        previous versions should be supported automatically, so we don't need
@@ -654,7 +654,7 @@ bool Context::tryCreate() {
     std::unordered_map<std::string, Extension> futureExtensions;
     for(std::size_t i = future; i != versions.size(); ++i)
         for(const Extension& extension: Extension::extensions(versions[i]))
-            futureExtensions.emplace(extension._string, extension);
+            futureExtensions.emplace(extension.string(), extension);
 
     /* Check for presence of future and vendor extensions */
     const std::vector<std::string> extensions = extensionStrings();
@@ -662,7 +662,7 @@ bool Context::tryCreate() {
         const auto found = futureExtensions.find(extension);
         if(found != futureExtensions.end()) {
             _supportedExtensions.push_back(found->second);
-            _extensionStatus.set(found->second._index);
+            _extensionStatus.set(found->second.index());
         }
     }
 
@@ -672,7 +672,7 @@ bool Context::tryCreate() {
     /* Initialize required versions from extension info */
     for(const auto version: versions)
         for(const Extension& extension: Extension::extensions(version))
-            _extensionRequiredVersion[extension._index] = extension._requiredVersion;
+            _extensionRequiredVersion[extension.index()] = extension.requiredVersion();
 
     /* Setup driver workarounds (increase required version for particular
        extensions), see Implementation/driverWorkarounds.cpp */
@@ -698,7 +698,7 @@ bool Context::tryCreate() {
         std::unordered_map<std::string, Extension> allExtensions{std::move(futureExtensions)};
         for(std::size_t i = 0; i != future; ++i)
             for(const Extension& extension: Extension::extensions(versions[i]))
-                allExtensions.emplace(extension._string, extension);
+                allExtensions.emplace(extension.string(), extension);
 
         /* Disable extensions that are known and supported and print a message
            for each */
@@ -707,7 +707,7 @@ bool Context::tryCreate() {
             /** @todo Error message here? I should not clutter the output at this point */
             if(found == allExtensions.end()) continue;
 
-            _extensionRequiredVersion[found->second._index] = Version::None;
+            _extensionRequiredVersion[found->second.index()] = Version::None;
             Debug{output} << "   " << extension;
         }
     }

src/Magnum/Context.h

@@ -64,6 +64,9 @@ class MAGNUM_EXPORT Extension {
         /** @brief All extensions for given OpenGL version */
         static const std::vector<Extension>& extensions(Version version);
 
+        /** @brief Internal unique extension index */
+        constexpr std::size_t index() const { return _index; }
+
         /** @brief Minimal version required by this extension */
         constexpr Version requiredVersion() const { return _requiredVersion; }
 
@@ -74,10 +77,6 @@ class MAGNUM_EXPORT Extension {
         constexpr const char* string() const { return _string; }
 
     private:
-        #ifndef DOXYGEN_GENERATING_OUTPUT /* https://bugzilla.gnome.org/show_bug.cgi?id=776986 */
-        friend Context;
-        #endif
-
         std::size_t _index;
         Version _requiredVersion;
         Version _coreVersion;
@@ -541,7 +540,7 @@ class MAGNUM_EXPORT Context {
          *      @ref MAGNUM_ASSERT_EXTENSION_SUPPORTED()
          */
         bool isExtensionSupported(const Extension& extension) const {
-            return isVersionSupported(_extensionRequiredVersion[extension._index]) && _extensionStatus[extension._index];
+            return isVersionSupported(_extensionRequiredVersion[extension.index()]) && _extensionStatus[extension.index()];
         }
 
         /**
@@ -574,7 +573,7 @@ class MAGNUM_EXPORT Context {
          * as it does most operations in compile time.
          */
         bool isExtensionDisabled(const Extension& extension) const {
-            return isVersionSupported(extension._requiredVersion) && !isVersionSupported(_extensionRequiredVersion[extension._index]);
+            return isVersionSupported(extension.requiredVersion()) && !isVersionSupported(_extensionRequiredVersion[extension.index()]);
         }
 
         /**