example: Fix dmenu run command
window: Reserve swc_ for external API
example/wm: Fix calculation of x coordinate for window geometry
example/wm: Use ceil instead of round to calculate number of columns

Most monitors are wider than they are tall, so it makes sense to round
up.
Adapt to wld changes
swc.pc: Add datadir variable so clients can find swc.xml
compositor: Rename struct screen -> struct target
framebuffer_plane: Use struct swc_mode instead of drmModeModeInfo
Make sure framebuffer and cursor planes are positioned correctly
compositor: Replace render_target with target
screen: Reserve swc_ prefix for public API