GL: harden extension list tests.

This uncovers a few important issues, yay!

src/Magnum/GL/Test/ContextTest.cpp

@@ -23,6 +23,7 @@
     DEALINGS IN THE SOFTWARE.
 */
 
+#include <set>
 #include <sstream>
 #include <Corrade/TestSuite/Tester.h>
 #include <Corrade/Utility/DebugStl.h>
@@ -95,7 +96,11 @@ void ContextTest::makeCurrentNoOp() {
 void ContextTest::extensions() {
     const char* used[GL::Implementation::ExtensionCount]{};
 
-    /* Check that all extension indices are unique */
+    std::set<std::string> unique;
+
+    /* Check that all extension indices are unique, are in correct lists, are
+       not compiled on versions that shouldn't have them, are listed just once
+       etc. */
     for(Version version: {
         #ifndef MAGNUM_TARGET_GLES
         Version::GL300,
@@ -133,6 +138,39 @@ void ContextTest::extensions() {
             }
 
             used[e.index()] = e.string();
+            if(!unique.insert(e.string()).second) {
+                Error{} << "Extension" << e.string() << "listed more than once";
+                CORRADE_VERIFY(false);
+            }
+
+            CORRADE_VERIFY(Int(e.coreVersion()) >= Int(e.requiredVersion()));
+            if(e.coreVersion() != version
+                #if defined(MAGNUM_TARGET_GLES2) && defined(MAGNUM_TARGET_WEBGL)
+                /* These two are replaced by EXT_color_buffer_float for 2.0,
+                   but aren't core in WebGL 2 */
+                && e.index() != Extensions::EXT::color_buffer_half_float::Index
+                && e.index() != Extensions::WEBGL::color_buffer_float::Index
+                #endif
+            ) {
+                Error{} << "Extension" << e.string() << "should have core version"
+                    << version << "but has" << e.coreVersion();
+                CORRADE_VERIFY(false);
+            }
+
+            #ifdef MAGNUM_TARGET_GLES2
+            if(e.requiredVersion() != Version::GLES200) {
+                Error{} << "Extension" << e.string() << "should have required version"
+                    << Version::GLES200 << "but has" << e.requiredVersion();
+                CORRADE_VERIFY(false);
+            }
+            #endif
+
+            #if defined(MAGNUM_TARGET_GLES) && !defined(MAGNUM_TARGET_GLES2)
+            if(e.coreVersion() == Version::GLES300) {
+                Error{} << "Extension" << e.string() << "has core version" << e.coreVersion() << "on a GLES3 build -- it shouldn't be present at all";
+                CORRADE_VERIFY(false);
+            }
+            #endif
         }
     }