framework/init: default to VK_LAYER_KHRONOS_validation for validation layer

framework: relax assertion on Assimp's primitives

autogen.sh: add CONFIGURE_FLAGS to conf_flags

framework: handle computed sem_index not being swap_chain_index

framework: add full wayland support on sdl platform

framework/platform: use SDL_GetError with some sdl calls

init: don't require VK_KHR_maintenance1

image: automatically convert RGB images to RGBA if needed

descriptor: increase maximum number of sets

vkdf: python2 is EOL, use python3 instead

vkdf: fix build errors on undefined VK_DYNAMIC_STATE_RANGE_SIZE

framework/scene: add gpointer cast

demos/cpu-particle-source: add a CPU particle source demo

framework/cpu-particle-source: add a CPU particle source

README update