GL: this array sizing was weird.

For some reason it had the full set of elements on ES2 as well, i.e.
something that definitely shouldn't be the case. Unfortunately C doesn't
warn if the array isn't initialized for all elements, setting the
remaining to 0, so this was uncaught until now. Now if I accidentally
make the array too small somewhere, it'll blow up inside
BufferState.cpp.

src/Magnum/GL/Implementation/BufferState.h

@@ -43,12 +43,12 @@ namespace Magnum { namespace GL { namespace Implementation {
 
 struct BufferState {
     enum: std::size_t {
-        #ifndef MAGNUM_TARGET_WEBGL
+        #ifdef MAGNUM_TARGET_GLES2 /* ES2 and WebGL 1 */
+        TargetCount = 2+1,
+        #elif defined(MAGNUM_TARGET_WEBGL) /* WebGL 2 */
+        TargetCount = 8+1,
+        #else /* ES3 and desktop */
         TargetCount = 13+1
-        #elif !defined(MAGNUM_TARGET_GLES2) && defined(MAGNUM_TARGET_WEBGL)
-        TargetCount = 8+1
-        #else
-        TargetCount = 2+1
         #endif
     };