exynos5410: libcamera: fix include guards
exynos5410: libcamera: fix preview stub responses
exynos5410: libcamera: add setPreviewWindow function
exynos5410: libcamera: allow retrieval of preview buffer count
squish into previous
exynos5410: libcamera: add preview thread skeleton
exynos5410: libcamera: partially set up preview buffers