clients/simple-dmabuf-*: Increase buffer limit to four

In certain situations these clients crash a lot due to the low
buffer limit. Four buffers is also what EGL allows without blocking
and what is arguably the upper limit of what a compositor should
demand.

Signed-off-by: Robert Mader <robert.mader@collabora.com>

clients/simple-dmabuf-egl.c

@@ -131,7 +131,7 @@ struct buffer {
 	int release_fence_fd;
 };
 
-#define NUM_BUFFERS 3
+#define NUM_BUFFERS 4
 
 struct window {
 	struct display *display;

clients/simple-dmabuf-feedback.c

@@ -47,7 +47,7 @@
 #include <GLES2/gl2.h>
 #include <GLES2/gl2ext.h>
 
-#define NUM_BUFFERS 3
+#define NUM_BUFFERS 4
 
 /* We have to hack the DRM-backend to pretend that planes of the underlying
  * hardware don't support this format. If you change the value of this constant,

clients/simple-dmabuf-v4l.c

@@ -127,7 +127,7 @@ struct buffer {
 	int data_offsets[VIDEO_MAX_PLANES];
 };
 
-#define NUM_BUFFERS 3
+#define NUM_BUFFERS 4
 
 struct window {
 	struct display *display;