bmp/decoder: avoid seeking where possible
bmp/decoder: avoid seeking where possible

seeking causes BufReader to drop its contents, which is bad for
performance.

Before:
test bench_load_bmp_16bit ... bench:     158,850 ns/iter (+/- 5,128)
test bench_load_bmp_24bit ... bench:     295,931 ns/iter (+/- 12,881)
test bench_load_bmp_32bit ... bench:   1,535,184 ns/iter (+/- 21,693)

After:
test bench_load_bmp_16bit ... bench:     149,930 ns/iter (+/- 6,964)
test bench_load_bmp_24bit ... bench:     265,234 ns/iter (+/- 12,579)
test bench_load_bmp_32bit ... bench:     343,867 ns/iter (+/- 20,745)

mbrubeck/image,PistonDevelopers/image,nwin/rust-image,PistonDevelopers/image