Add API to enable multisampling in GdkGLContext

A few people have been asking for this, both offline and online, in order to render with MSAA

When selecting features for a GL context, we need to add the appropriate attributes:

• EGL_SAMPLE_BUFFERS and EGL_SAMPLES for EGL
• GLX_SAMPLE_BUFFERS and GLX_SAMPLES for GLX
• WGL_SAMPLE_BUFFERS_ARB and WGL_SAMPLES_ARB for Windows

Since those attributes take a parameter, we're going to need API in GdkGLContext similar to how we require a specific GL version; for instance:

void gdk_gl_context_set_multisampling (GdkGLContext *context,
                                       int           n_buffers);
int  gdk_gl_context_get_multisampling (GdkGLContext *context);

with:

• n_buffers == 0 disabling multisampling explicitly
• n_buffers > 0 enabling multisampling explicitly, with the given number of buffers
• n_buffers < 0 using the implementation's default

If n_buffers > 0 we will add the [ SAMPLE_BUFFERS, 1 ] and [ SAMPLES, n_buffers ] attributes to the GL context creation.