render/vulkan: don't use UNDEFINED layout for imported DMA-BUFs

UNDEFINED when used as source layout means that the contents of
the underlying memory becomes undefined. This isn't what we want
here: we don't want to mutate the imported pixel data.

The Vulkan spec isn't really clear what the proper value should be
here, but after discussing with driver developers [1] it seems like
UNDEFINED isn't the right one. The recommendation is to use GENERAL
instead.

[1]: https://github.com/ValveSoftware/gamescope/issues/356

render/vulkan/pass.c

@@ -187,9 +187,7 @@ static bool render_pass_submit(struct wlr_render_pass *wlr_pass) {
 	size_t idx = 0;
 	uint32_t render_wait_len = 0;
 	wl_list_for_each_safe(texture, tmp_tex, &renderer->foreign_textures, foreign_link) {
-		VkImageLayout src_layout = VK_IMAGE_LAYOUT_GENERAL;
 		if (!texture->transitioned) {
-			src_layout = VK_IMAGE_LAYOUT_UNDEFINED;
 			texture->transitioned = true;
 		}
 
@@ -199,7 +197,7 @@ static bool render_pass_submit(struct wlr_render_pass *wlr_pass) {
 			.srcQueueFamilyIndex = VK_QUEUE_FAMILY_FOREIGN_EXT,
 			.dstQueueFamilyIndex = renderer->dev->queue_family,
 			.image = texture->image,
-			.oldLayout = src_layout,
+			.oldLayout = VK_IMAGE_LAYOUT_GENERAL,
 			.newLayout = VK_IMAGE_LAYOUT_SHADER_READ_ONLY_OPTIMAL,
 			.srcAccessMask = 0, // ignored anyways
 			.dstAccessMask = VK_ACCESS_SHADER_READ_BIT,

render/vulkan/texture.c

@@ -842,7 +842,7 @@ void wlr_vk_texture_get_image_attribs(struct wlr_texture *texture,
 	attribs->image = vk_texture->image;
 	attribs->format = vk_texture->format->vk;
 	attribs->layout = vk_texture->transitioned ?
-		VK_IMAGE_LAYOUT_SHADER_READ_ONLY_OPTIMAL : VK_IMAGE_LAYOUT_UNDEFINED;
+		VK_IMAGE_LAYOUT_SHADER_READ_ONLY_OPTIMAL : VK_IMAGE_LAYOUT_GENERAL;
 }
 
 bool wlr_vk_texture_has_alpha(struct wlr_texture *texture) {