u_blitter/stencil: take dstbox x/y into accounts for dst fb width
When setting the dst framebuffer width height, it might be silly to constrain this beyond the dst resource, but at least constrain it correctly to take account of x/y offsets.
This fixes some uses of this as a fallback for zink with GTF-GL46.gtf30.GL3Tests.framebuffer_blit.framebuffer_blit_functionality_stencil_blit
Fixes: b4c07a8a ("gallium/util: allow scaling blits for stencil-fallback")