Corrected the formulae to calculate the location of offset pixels

This is regarding #899 issue.

In existing code, following formulae is used to calculate row and column offset:
`row = r + <int>(sin(angle) * distance + 0.5)`
`col = c + <int>(cos(angle) * distance + 0.5)`
that give offset of (1, 0) for both `pi/2` and `3pi/4`. It should give offset of (1, 0) for `pi/2` and that of (1, -1) for `3pi/4`.

I modified the formulae as:
`row = r + <int>floor(sin(angle) * distance + 0.5)`
`col = c + <int>floor(cos(angle) * distance + 0.5)`
Now it's giving the correct offset.

In offset (x, y), x corresponds to row offset and y corresponds to column offset.