libply-splash-core: Add a force argument to ply_renderer_open ()

libply-splash-core: Add a force argument to ply_renderer_open ()

Sometimes a renderer plugin may want to refuse to open a device
because it has suboptimal settings, like e.g. an EFI framebuffer
based simpledrm when the EFI firmware has set the resolution to
800x600 instead of the native panel resolution.

In such a case it might be better to wait for another better
/dev/dri/card# device to show up.

This skipping of devices by renderer plugins needs to be configurable
in case the timeout hits, or the user wishes to override things.

Add a force argument to ply_renderer_open () to allow overriding
this behavior. User can force using simpledrm by passing
plymouth.use-simpledrm=2 on the kernel commandline or by setting
UseSimpledrm=2 in the config-file.

This flag is passed to the renderer plugin's query_device () method
as that is the best place for the renderer plugin to determine
a device's usability.

Signed-off-by: Hans de Goede <hdegoede@redhat.com>