splash: Select splash image based on native resolution

[MilhouseVH]

With this PR, init will parse fbset for the currently set native resolution. It will then use a matching splash image if one exists - if not, the 1080 image will be used.

Currently this PR is missing a 720p splash image, I'll add one when it becomes available. The size of a 720p PNG splash should be 80KB 8KB or less. If not too large (in terms of file size) I may also add a 4K (3840x2160) image.

With this PR, the vast majority of native resolutions - 1080p, 720p, and (1024x)768 - will be catered for, and shouldn't need the artwork to be resized by plymouth-lite.

Any other "corner case" resolutions will use the 1080 artwork, and plymouth-lite (with or without bilinear, #353) will resize to the display resolution.

Extending this in future to support additional native resolution artwork should be trivial - just add the splash_<vertical resolution>.png artwork.

As an extra, this PR adds support for SWITCH_FRAMEBUFFER, allowing custom frame buffer resolutions to be configured from the command line. For example, adding setfbres=720 would force fbset -g 1280 720 1280 720 32 (setfbres=1080 is also supported). An entirely custom resolution can be set with (for example) setfbres=640,480,640,480,32.

[MilhouseVH]

Custom distributions should note that the filenames of the splash images are changing with this PR:

splash_full.png to splash_1080.png
splash_1024.png to splash_768.png

Distributions must at the very least provide a splash image named splash_1080.png.

[vpeter4]

Little more optimzed
vres="$(fbset | awk '/geometry/ { print $3 }')"

[MilhouseVH]

@vpeter4 thanks, updated.

[MilhouseVH]

Updated with 1280x720, 1920x1200 and 3840x2160 artwork.:

-rw-rw-r-- 1 neil neil  9278 May 22 20:03 splash-1080.png
-rw-rw-r-- 1 neil neil  9504 May 22 20:08 splash-1200.png
-rw-rw-r-- 1 neil neil 23284 May 22 20:08 splash-2160.png
-rw-rw-r-- 1 neil neil  6073 May 22 20:08 splash-720.png
-rw-rw-r-- 1 neil neil  5972 May 22 20:08 splash-768.png