loader: show lt error

despite it almost always being the /informative/ "file not found" message.
loader: always search in build-dir

It opens up for security issues with arbitrary code execution but that is already possible by overriding SLIDESHOW_PLUGIN_DIR env var.
transition: more accurate preview

now it goes from 0 -> 1, swaps and next 0 -> 1 again instead of 0 -> 1 -> 0.
graphics: increment during swap

instead of during loading. Makes it work during transition preview
Merge commit 'f6eaead'
noise: adding noise transition

not very good yet
Merge branch 'noise_transition'
build: not needed, covered by later test