Improve vt(4) splash screen support

At the moment (2018-09-11) vt(4) has very limited support for splash screens.

The goal of this project is to extend it a little bit. It should be possible to:

• load a specified BMP file to be displayed as splash screen
• control display colors via kernel tunables

A stretched goal is to support bitmaps with the color depth of more than 1.

• splash(4)
• Newcons
• BMP file format (Wikipedia)

• Let's improve splash screen support in vt(4) (freebsd-hackers@, 2018-09-11)

Commit adding some splash screen support to vt(4): 2ecca071d5eccf38f9a5fd7a03052b261b7bd7d2

sc(4) supports 8-bit colors.

Interesting functions:

• vd_bitblt_bmp
• vt_fb_bitblt_bitmap (vt_fb.c)
• vt_fb_setpixel: it seems to be the function which is capable of setting pixel colors in a console controlled by vt(4).

There's a function called vtterm_splash in vt_core.c, which displays a boot splash when the kernel is compiled with DEV_SPLASH option, textmode is not activated and a muted booting was requested by the user (for example via specifying -m to boot(8)).

Also the following part of the vtterm_splash function:

switch (vt_logo_depth) {
case 1:
        /* XXX: Unhardcode colors! */
        vd->vd_driver->vd_bitblt_bmp(vd, vd->vd_curwindow,
            vt_logo_image, NULL, vt_logo_width, vt_logo_height,
            left, top, TC_WHITE, TC_BLACK);

^{Source: vt_core.c}

suggests that:

1. Monochrome colors are the only one supported at the moment.
2. It should not be complicated to add support for 4-bit and 8-bit colors.
   • Actually, there is an sc(4) file, which has a very similar switch construct handling various color depth configurations.
     • What's the name of the sc(4) function with that switch case?
       • There's no such function in sc(4). There is bmp_SetPix in sys/dev/fb/splash_bmp.c but it does not help much as it directly sets video bits. Its signature looks like this: static void bmp_SetPix(BMP_INFO *info, int x, int y, u_char val).

Other questions regarding the vtterm_splash function:

1. {o} What is the VDF_SPLASH flag inside vd->vd_flags for?
2. {o} How to load a BMP file into a structure accepted by functions displaying pixels? Could it be that the splash(4) driver provides all the required functions to do so? Ideal scenario:
   1. The splash(4) driver reads the bitmap file that was specified in loader.conf(5) and converts it to a structure accepted by internal vt(4) functions.
   2. vt(4) gets the structure from splash(4).
   3. vt(4) displays the bitmap.
3. {o} How does the vd->vd_driver->vd_bitblt_bmp function work?
   • struct vt_driver seems to be an interface for different hardware have to implement in order to use the vt(4) driver. vd_bitblt_bmp happens to be one of the drawing functions. It has various implementations like ofwfb_bitblt_bitmap, vga_bitblt_bitmap or vt_fb_bitblt_bitmap.

At the moment the vt(4) driver stores logos in unsigned char structures (see sys/dev/vt/logo/logo_beastie.c), which roughly correspond to the u_char *data member of the BMP_INFO structure (see sys/dev/fb/splash_bmp.c).

typedef struct
{
    int     width,height;           /* image dimensions */
    int     swidth,sheight;         /* screen dimensions for the current mode */
    u_char  depth;                  /* image depth (1, 4, 8, 24 bits) */
    u_char  sdepth;                 /* screen depth (1, 4, 8 bpp) */
    int     ncols;                  /* number of colours */
    u_char  palette[256][3];        /* raw palette data */
    u_char  format;                 /* one of the BI_* constants above */
    u_char  *data;                  /* pointer to the raw data */
    u_char  *index;                 /* running pointer to the data while drawing */
    u_char  *vidmem;                /* video memory allocated for drawing */
    video_adapter_t *adp;
    int             bank;
} BMP_INFO;

^{Source: sys/dev/fb/splash_bmp.c}

const unsigned char vt_beastie_vga16[] = {
        0x16, 0x00, 0x62, 0x16, 0x88, 0x03, 0x80, 0x16, 0x00, 0x23, 0x88, 0x80,
        0x00, 0x00, 0x08, 0x88, 0x16, 0x00, 0x21, 0x08, 0x16, 0x00, 0x06, 0x88,
        0x16, 0x00, 0x20, 0x80, 0x16, 0x00, 0x06, 0x08, 0x80, 0x16, 0x00, 0x1e,

^{Source: sys/dev/vt/logo/logo_beastie.c}

• DIB: device independent bitmap