glDrawBuffer() is not available in ES3.

Using glDrawBuffers() also in this case.

src/AbstractFramebuffer.cpp

@@ -219,8 +219,12 @@ void AbstractFramebuffer::drawBufferImplementationDefault(GLenum buffer) {
     /** @todo Re-enable when extension wrangler is available for ES2 */
     #ifndef MAGNUM_TARGET_GLES2
     bindInternal(drawTarget);
+    #ifndef MAGNUM_TARGET_GLES3
     glDrawBuffer(buffer);
     #else
+    glDrawBuffers(1, &buffer);
+    #endif
+    #else
     static_cast<void>(buffer);
     #endif
 }

src/DefaultFramebuffer.h

@@ -306,7 +306,8 @@ class MAGNUM_EXPORT DefaultFramebuffer: public AbstractFramebuffer {
          * framebufferbuffer is not currently bound, it is bound before the
          * operation.
          * @see mapForRead(), @fn_gl{BindFramebuffer}, @fn_gl{DrawBuffer} or
-         *      @fn_gl_extension{FramebufferDrawBuffer,EXT,direct_state_access}
+         *      @fn_gl_extension{FramebufferDrawBuffer,EXT,direct_state_access},
+         *      @fn_gl{DrawBuffers} in OpenGL ES 3.0
          * @requires_gles30 Draw attachments for default framebuffer are
          *      available only in OpenGL ES 3.0.
          */

src/Framebuffer.h

@@ -262,7 +262,8 @@ class MAGNUM_EXPORT Framebuffer: public AbstractFramebuffer {
          * framebufferbuffer is not currently bound, it is bound before the
          * operation.
          * @see mapForRead(), @fn_gl{BindFramebuffer}, @fn_gl{DrawBuffer} or
-         *      @fn_gl_extension{FramebufferDrawBuffer,EXT,direct_state_access}
+         *      @fn_gl_extension{FramebufferDrawBuffer,EXT,direct_state_access},
+         *      @fn_gl{DrawBuffers} in OpenGL ES 3.0
          * @requires_gles30 %Extension @es_extension2{NV,draw_buffers,GL_NV_draw_buffers}
          */
         inline Framebuffer* mapForDraw(DrawAttachment attachment) {