image: fix interface

[sago35]

This PR fixes the interface that was discussed in #363.

The test is not OK yet, but it is the same as the Go standard test result, so we won't deal with it here.

sago3@B450GT3 MINGW64 ~/dev/src/tinygo.org/x/drivers (fix-interface)
$ cd image/png/ && tinygo test
FAIL    tinygo.org/x/drivers/image/png  0.156s
FAIL

sago3@B450GT3 MINGW64 ~/dev/src/tinygo.org/x/drivers/image/png (fix-interface)
$ cd ../jpeg/ && tinygo test
ld.lld: error: undefined symbol: time.startTimer
>>> referenced by C:\Go\src\time\sleep.go:96
>>>               C:\Users\sago3\AppData\Local\Temp\tinygo2750643149\main.o:(tinygo.org/x/drivers/image/jpeg.TestLargeImageWithShortData)
failed to run tool: ld.lld
error: failed to link C:\Users\sago3\AppData\Local\Temp\tinygo2750643149\main.exe: exit status 1
FAIL

[deadprogram]

Thanks for the fix here @sago35 now merging.