ext_external_objects: Vulkan framework transitions fix, corrections in vk_semaphores
In vk_clear_color we need to transition both color and depth buffers at the end of renderpass because the last known color attachment layout is VK_LAYOUT_COLOR_ATTACHMENT_OPTIMAL, and not VK_LAYOUT_GENERAL we used to clear the image.
Also, vk-semaphores test doesn't need to clear the depth bit in OpenGL (removed).
Edited by Eleni Maria Stea