Mesh: warn about improper buffer binding also on Emscripten.

It seems that rebinding is forbidden by law also in WebGL. Also updated
Best Practices docs to note that.

doc/best-practices.dox

@@ -57,17 +57,17 @@ information.
 
 - [Best practices for 3D graphics](https://developers.google.com/native-client/beta/devguide/coding/3D-graphics#best-practices)
 
-@subsubsection best-practices-nacl-buffer-types Native Client requires unique buffer binding
+@subsection best-practices-web-buffer-types Native Client and Emscripten require unique buffer binding
 
-As noted in the above link, buffers in NaCl implementation need to be bound
-only to one unique target, i.e., @ref Buffer bound to @ref Buffer::Target::Array
-cannot be later rebound to @ref Buffer::Target::ElementArray. However, %Magnum
-by default uses any sufficient target when binding the buffer internally (e.g.
-for setting data or copying). To avoid this, set target hint to desired target,
-either in constructor or using @ref Buffer::setTargetHint().
+As noted in the above link, buffers in NaCl implementation and and also in
+WebGL need to be bound only to one unique target, i.e., @ref Buffer bound to
+@ref Buffer::Target::Array cannot be later rebound to @ref Buffer::Target::ElementArray.
+However, %Magnum by default uses any sufficient target when binding the buffer
+internally (e.g. for setting data or copying). To avoid this, set target hint
+to desired target, either in constructor or using @ref Buffer::setTargetHint().
 
 To ease up the development, @ref Mesh checks proper target hint when adding
-vertex and index buffers.
+vertex and index buffers in both Native Client and Emscripten.
 
 @section best-practices-hw Hardware-specific
 

src/Mesh.cpp

@@ -138,7 +138,7 @@ Mesh& Mesh::operator=(Mesh&& other) noexcept {
 }
 
 Mesh& Mesh::setIndexBuffer(Buffer& buffer, GLintptr offset, IndexType type, UnsignedInt start, UnsignedInt end) {
-    #ifdef CORRADE_TARGET_NACL
+    #if defined(CORRADE_TARGET_NACL) || defined(CORRADE_TARGET_EMSCRIPTEN)
     CORRADE_ASSERT(buffer.targetHint() == Buffer::Target::ElementArray,
         "Mesh::setIndexBuffer(): the buffer has unexpected target hint, expected" << Buffer::Target::ElementArray << "but got" << buffer.targetHint(), *this);
     #endif
@@ -265,7 +265,7 @@ void Mesh::destroyImplementationVAO() {
 }
 
 void Mesh::attributePointerImplementationDefault(const Attribute& attribute) {
-    #ifdef CORRADE_TARGET_NACL
+    #if defined(CORRADE_TARGET_NACL) || defined(CORRADE_TARGET_EMSCRIPTEN)
     CORRADE_ASSERT(attribute.buffer->targetHint() == Buffer::Target::Array,
         "Mesh::addVertexBuffer(): the buffer has unexpected target hint, expected" << Buffer::Target::Array << "but got" << attribute.buffer->targetHint(), );
     #endif
@@ -274,7 +274,7 @@ void Mesh::attributePointerImplementationDefault(const Attribute& attribute) {
 }
 
 void Mesh::attributePointerImplementationVAO(const Attribute& attribute) {
-    #ifdef CORRADE_TARGET_NACL
+    #if defined(CORRADE_TARGET_NACL) || defined(CORRADE_TARGET_EMSCRIPTEN)
     CORRADE_ASSERT(attribute.buffer->targetHint() == Buffer::Target::Array,
         "Mesh::addVertexBuffer(): the buffer has unexpected target hint, expected" << Buffer::Target::Array << "but got" << attribute.buffer->targetHint(), );
     #endif