fix(bmp): correct bpp shift and prevent overflow

[tachibana-shin]

• Correct bit shift to calculate bytes per pixel
• Change tc to long to prevent integer overflow

Fix: bpp>>8 → bpp>>3 in bmp_clear_outside_crop_border

Bug: bytes_per_pix was always 0, causing all memset() calls in this function to be no-ops (writing 0 bytes). Color bitmap crop borders were never cleared to white.

Root cause: bpp is bits per pixel (8 or 24, per willus.h:517). Dividing by 256 (>>8) is wrong — should be dividing by 8 (>>3) to convert bits to bytes.

Evidence:

1. Struct definition (willus.h:517):

   int bpp;  /* Bits per pixel (only 8 or 24 allowed) */

2. Same function — bytes_per_pix is used to calculate memset size (k2bmp.c:244-280):

p = bmp_rowptr_from_top(src, i);
memset(p, 255, n * bytes_per_pix);  // left border
// ...
memset(p, 255, src->width * bytes_per_pix);  // top/bottom rows

With bytes_per_pix=0, memset writes 0 bytes — color borders are never cleared.
4. Consistent with bmp_bytewidth (bmp.c:1410):

return(bmp->bpp==24 ? bmp->width*3 : bmp->width);

Same conversion: bpp 24 → 3 bytes, bpp 8 → 1 byte.
Before: bpp>>8: 8→0, 24→0 (always 0, wrong)
After: bpp>>3: 8→1, 24→3 (correct bytes per pixel)

---

This change is 

[Frenzie]

Alright, sounds good.