set blending option once, before the loop

[N-R-K]

instead of setting them everytime in the loop, it's sufficient to set it just once before entering the loop.

also remove imlib_context_set_angle() since that is a text related setting and has no effect on imlib_blend_image_onto_image().

[N-R-K]

remove imlib_context_set_angle() since that is a text related setting

I noticed this when trying to link against imlib2 built with --disable-text and saw the error:

src/scrot.c:1016: undefined reference to `imlib_context_set_angle'

This was on a local branch with scrot's --notes feature removed.

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Everything should still work fine, I think. But I do not use a compositor so I could not test, since -k requires compositor.

If someone can test and confirm everything to be working, then I can merge this.

[N-R-K]

I just removed this check and tried scrot -k and everything worked perfectly fine without running any compositor.

https://github.com/resurrecting-open-source-projects/scrot/blob/5c2d6afe801ee36d0744e6e89c2cf4f8156dcff1/src/scrot.c#L832-L836

I don't think a compositor is needed for -k since we redirect all the sub-windows manually here:

https://github.com/resurrecting-open-source-projects/scrot/blob/5c2d6afe801ee36d0744e6e89c2cf4f8156dcff1/src/scrot.c#L866

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Here's the resulting screenshot:

[N-R-K]

Tested this PR with that check removed and everything seems to be working fine.

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Also opened a new PR for removing the compositor check: #354. So let's move any discussion about that over to the new PR.