c/swapchain: Don't only give out the zero index

doc/changes/compositor/mr.359.1.md

@@ -0,0 +1,2 @@
+swapchain: Give out the oldset image index when a image is acquired. This logic
+can be made better, but will work for the good case.

src/xrt/compositor/main/comp_compositor.h

@@ -13,6 +13,7 @@
 #include "xrt/xrt_gfx_vk.h"
 
 #include "util/u_threading.h"
+#include "util/u_index_fifo.h"
 
 #include "main/comp_settings.h"
 #include "main/comp_window.h"
@@ -64,6 +65,12 @@ struct comp_swapchain
 	struct comp_compositor *c;
 
 	struct comp_swapchain_image images[XRT_MAX_SWAPCHAIN_IMAGES];
+
+	/*!
+	 * This fifo is used to always give out the oldest image to acquire
+	 * image, this should probably be made even smarter.
+	 */
+	struct u_index_fifo fifo;
 };
 
 enum comp_layer_type

src/xrt/compositor/main/comp_swapchain.c

@@ -31,8 +31,9 @@ swapchain_acquire_image(struct xrt_swapchain *xsc, uint32_t *index)
 	struct comp_swapchain *sc = comp_swapchain(xsc);
 
 	COMP_SPEW(sc->c, "ACQUIRE_IMAGE");
-	*index = 0;
-	return true;
+
+	// Returns negative on empty fifo.
+	return u_index_fifo_pop(&sc->fifo, index) >= 0;
 }
 
 static bool
@@ -52,6 +53,9 @@ swapchain_release_image(struct xrt_swapchain *xsc, uint32_t index)
 	struct comp_swapchain *sc = comp_swapchain(xsc);
 
 	COMP_SPEW(sc->c, "RELEASE_IMAGE");
+
+	u_index_fifo_push(&sc->fifo, index);
+
 	return true;
 }
 
@@ -273,6 +277,11 @@ comp_swapchain_create(struct xrt_compositor *xc,
 		}
 	}
 
+	// Prime the fifo
+	for (uint32_t i = 0; i < num_images; i++) {
+		u_index_fifo_push(&sc->fifo, i);
+	}
+
 
 	/*
 	 *