GL: advertise WEBGL_multi_draw only on Emscripten 2.0.0 and up.

src/Magnum/GL/AbstractShaderProgram.h

@@ -844,7 +844,9 @@ class MAGNUM_GL_EXPORT AbstractShaderProgram: public AbstractObject {
          * @m_class{m-doc-external} [ANGLE_multi_draw](https://chromium.googlesource.com/angle/angle/+/master/extensions/ANGLE_multi_draw.txt)
          * is present, and on WebGL if @webgl_extension{WEBGL,multi_draw} is
          * not present, the functionality is emulated using a sequence of
-         * @ref draw(MeshView&) calls.
+         * @ref draw(MeshView&) calls. Note that @webgl_extension{WEBGL,multi_draw}
+         * is only implemented since Emscripten 2.0.0, so it's not even
+         * advertised on older versions.
          *
          * If @gl_extension{ARB,vertex_array_object} (part of OpenGL 3.0),
          * OpenGL ES 3.0, WebGL 2.0, @gl_extension{OES,vertex_array_object} in

src/Magnum/GL/Implementation/MeshState.cpp

@@ -136,9 +136,16 @@ MeshState::MeshState(Context& context, ContextState& contextState, std::vector<s
             extensions.push_back(Extensions::WEBGL::multi_draw::string());
 
             /* The WEBGL extension uses the same entrypoints as the ANGLE
-               extension it was based on */
+               extension it was based on. Only available since 2.0.0:
+               https://github.com/emscripten-core/emscripten/pull/11650 */
+            #if __EMSCRIPTEN_major__*10000 + __EMSCRIPTEN_minor__*100 + __EMSCRIPTEN_tiny__ >= 20000
             multiDrawArraysImplementation = glMultiDrawArraysANGLE;
             multiDrawElementsImplementation = glMultiDrawElementsANGLE;
+            #else
+            /* In Context::setupDriverWorkarounds() we make sure the extension
+               is not even advertised, so this shouldn't be reached. */
+            CORRADE_INTERNAL_ASSERT_UNREACHABLE();
+            #endif
         }
         #endif
 

src/Magnum/GL/Implementation/driverSpecific.cpp

@@ -560,6 +560,16 @@ void Context::setupDriverWorkarounds() {
         _setRequiredVersion(EXT::disjoint_timer_query, None);
     #endif
 
+    #ifdef MAGNUM_TARGET_WEBGL
+    /* The WEBGL_multi_draw entrypoints are only available since Emscripten
+       2.0.0: https://github.com/emscripten-core/emscripten/pull/11650
+       However, the extension is advertised even on older versions and we have
+       no way to link to those entrypoints there. */
+    #if __EMSCRIPTEN_major__*10000 + __EMSCRIPTEN_minor__*100 + __EMSCRIPTEN_tiny__ < 20000
+    _setRequiredVersion(WEBGL::multi_draw, None);
+    #endif
+    #endif
+
     #undef _setRequiredVersion
 
     #ifndef MAGNUM_TARGET_GLES