Implement module-in-focus process cache

The standard pixelpipe cache offers *input* data for a module to be fully processed if all parameters including
blending stuff match.

This commit implements a single line cache per pixelpipe only effective for a module being in focus *and* doing
some blending.

Data are possibly read from this cache instead of the possibly costly module->process/_cl.
Possibly write to that cache if data were not valid before.

Fixes #18180

src/develop/imageop.c

@@ -2358,6 +2358,13 @@ void dt_iop_request_focus(dt_iop_module_t *module)
   dev->gui_module = module;
   dev->focus_hash = TRUE;
 
+  dt_free_align(dev->full.pipe->bcache_data);
+  dev->full.pipe->bcache_data = NULL;
+  dt_free_align(dev->preview_pipe->bcache_data);
+  dev->preview_pipe->bcache_data = NULL;
+  dt_free_align(dev->preview2.pipe->bcache_data);
+  dev->preview2.pipe->bcache_data = NULL;
+
   /* lets lose the focus of previous focus module*/
   if(out_focus_module)
   {