gl_x11: always request true color visual

This fixes issue #504. For some reason, glXChooseFBConfig() will return
a fbconfig with no associated visual. (I'm not sure if this allowed.
They don't always have a visual, but since GLX_X_RENDERABLE is set
and GLX_DRAWABLE_TYPE is (implicitly) set to GLX_WINDOW_BIT, why would
there be no visual?)

Even worse, a test program seems to show that a 16 bit fbconfig is
selected (instead of 24/32 bit), which doesn't sound nice at all. Since
there _are_ better fbconfigs available, glXChooseFBConfig() should
normally sort them by quality, and return the better ones first. It's
worth noting that this function should also prefer GLX_TRUE_COLOR
over anything else, although this comes last in the sort order.

Whatever is going on, requesting GLX_X_VISUAL_TYPE with GLX_TRUE_COLOR
seems to fix it.

video/out/gl_x11.c

@@ -218,6 +218,7 @@ static bool config_window_x11(struct MPGLContext *ctx, uint32_t d_width,
     int glx_attribs[] = {
         GLX_STEREO, False,
         GLX_X_RENDERABLE, True,
+        GLX_X_VISUAL_TYPE, GLX_TRUE_COLOR,
         GLX_RED_SIZE, 1,
         GLX_GREEN_SIZE, 1,
         GLX_BLUE_SIZE, 1,