evas/scale_sample: call alloca for the scanline buffer after clamping width

Summary:
this is already a risky call for larger scanlines, so use the clamped value
to further reduce the chance of blowing out the stack

Depends on D8839

Reviewers: cedric, segfaultxavi

Reviewed By: segfaultxavi

Subscribers: segfaultxavi, reviewers, committers

Tags: efl: rendering

Differential Revision: https://phab.enlightenment.org/D8840