Re-add glDrawRangeElements() workaround for Emscripten.

This reverts commit f6ba4111e1, which in
turn reverts commit 4ce2875262 from 2015.
Turns out glDrawRangeElements() *is* fixed now in Firefox, but is broken
in Emscripten because their function dependency handling doesn't work
correctly. Related PR: https://github.com/kripken/emscripten/pull/7112

Reverting this until the Emscripten PR is integrated and a version
released with this patch is widespread enough (assuming a year-long
delay could do).

doc/changelog.dox

@@ -198,8 +198,6 @@ See also:
     possible as well
 -   The @ref GL::BufferUsage parameter in @ref GL::Buffer::setData() is now
     optional, defaults to @ref GL::BufferUsage::StaticDraw
--   Re-enabled @fn_gl{DrawRangeElements} for WebGL 2 (see
-    [mosra/magnum#97](https://github.com/mosra/magnum/issues/97))
 -   Restored backwards compatibility to the templated @ref GL::Buffer::map()
     overload --- it was not possible to call it with @cpp void @ce template
     parameter

src/Magnum/GL/Mesh.cpp

@@ -411,7 +411,10 @@ void Mesh::drawInternal(Int count, Int baseVertex, Int instanceCount, GLintptr i
 
         /* Indexed mesh */
         } else {
-            #ifndef MAGNUM_TARGET_GLES2
+            /** @todo re-enable once https://github.com/kripken/emscripten/pull/7112
+                is merged and Emscripten versions with this change are
+                widespread enough */
+            #if !defined(MAGNUM_TARGET_GLES2) && !defined(MAGNUM_TARGET_WEBGL)
             /* Indexed mesh with specified range */
             if(indexEnd) {
                 glDrawRangeElements(GLenum(_primitive), indexStart, indexEnd, count, GLenum(_indexType), reinterpret_cast<GLvoid*>(indexOffset));