render/vulkan: align staging buffers for texture upload

vkCmdCopyBufferToImage requires that the buffer offset be a multiple
of the texel block size, which for single plane uncompressed formats
is the same as the number of bytes per pixel. This commit adds an
alignment parameter to vulkan_get_stage_span which ensures that the
provided span (and the sequence of image copy operations derived which
use it) have this alignment.

render/vulkan/renderer.c

@@ -200,7 +200,7 @@ static void release_stage_allocations(struct wlr_vk_renderer *renderer) {
 }
 
 struct wlr_vk_buffer_span vulkan_get_stage_span(struct wlr_vk_renderer *r,
-		VkDeviceSize size) {
+		VkDeviceSize size, VkDeviceSize alignment) {
 	// try to find free span
 	// simple greedy allocation algorithm - should be enough for this usecase
 	// since all allocations are freed together after the frame
@@ -215,6 +215,10 @@ struct wlr_vk_buffer_span vulkan_get_stage_span(struct wlr_vk_renderer *r,
 		}
 
 		assert(start <= buf->buf_size);
+
+		// ensure the proposed start is a multiple of alignment
+		start += alignment - 1 - ((start + alignment - 1) % alignment);
+
 		if (buf->buf_size - start < size) {
 			continue;
 		}

render/vulkan/texture.c

@@ -73,7 +73,7 @@ static bool write_pixels(struct wlr_vk_texture *texture,
 	}
 
 	// get staging buffer
-	struct wlr_vk_buffer_span span = vulkan_get_stage_span(renderer, bsize);
+	struct wlr_vk_buffer_span span = vulkan_get_stage_span(renderer, bsize, bytespb);
 	if (!span.buffer || span.alloc.size != bsize) {
 		wlr_log(WLR_ERROR, "Failed to retrieve staging buffer");
 		free(copies);