Buffer::mapAlignment() requires ARB_map_buffer_alignment.

Return 0 if the extension is not supported.

src/Magnum/Buffer.cpp

@@ -80,6 +80,9 @@ void Buffer::initializeContextBasedFunctionality(Context& context) {
 
 #ifndef MAGNUM_TARGET_GLES
 Int Buffer::minMapAlignment() {
+    if(!Context::current()->isExtensionSupported<Extensions::GL::ARB::map_buffer_alignment>())
+        return 0;
+
     GLint& value = Context::current()->state().buffer->minMapAlignment;
 
     if(value == 0)

src/Magnum/Buffer.h

@@ -422,7 +422,8 @@ class MAGNUM_EXPORT Buffer: public AbstractObject {
          * @brief Minimal supported mapping alignment
          *
          * The result is cached, repeated queries don't result in repeated
-         * OpenGL calls.
+         * OpenGL calls. If extension @extension{ARB,map_buffer_alignment} is
+         * not available, returns `0`.
          * @see @ref map(), @fn_gl{Get} with @def_gl{MIN_MAP_BUFFER_ALIGNMENT}
          * @requires_gl No minimal value is specified for OpenGL ES.
          */