glamor: Hide new DRI behind Option "Debug" "dmabuf_capable"

... for xfree86, at least for now. Things appear to work for Xwayland
but not yet for modesetting. Hopefully we can fix that before 1.20 but
in the meantime this makes testing both paths easier than a rebuild.

Signed-off-by: Adam Jackson <ajax@redhat.com>

glamor/glamor_egl.c

@@ -35,6 +35,7 @@
 #include <sys/ioctl.h>
 #include <errno.h>
 #include <xf86.h>
+#include <xf86Priv.h>
 #include <xf86drm.h>
 #define EGL_DISPLAY_NO_X_MESA
 
@@ -960,7 +961,8 @@ glamor_egl_init(ScrnInfoPtr scrn, int fd)
                                 "EGL_EXT_image_dma_buf_import") &&
         epoxy_has_egl_extension(glamor_egl->display,
                                 "EGL_EXT_image_dma_buf_import_modifiers"))
-        glamor_egl->dmabuf_capable = TRUE;
+        glamor_egl->dmabuf_capable = !!strstr(xf86Info.debug,
+                                              "dmabuf_capable");
 #endif
 
     glamor_egl->saved_free_screen = scrn->FreeScreen;

hw/xfree86/man/xorg.conf.man

@@ -498,6 +498,9 @@ The options recognised by this section are:
 .BI "Option \*qDebug\*q  \*q" string \*q
 This comma-separated list provides a way to control various debugging switches
 from the config file.
+At the moment the only defined value is
+.B dmabuf_capable
+which instructs glamor to enable some unstable buffer management code.
 .TP 7
 .BI "Option \*qDefaultServerLayout\*q  \*q" layout\-id \*q
 This specifies the default