Revert "Quell divide-by-zero warning"

This reverts commit 07b04775ff.

This code is performance-critical: adding a branch for every pixel in
the surface isn't a good idea. @GregoryLundberg was fine with reverting
the commit, as well. The same is true for the next two reverts.

Fixing Coverity Scan warnings is good and all, but we shouldn't
sacrifice performance for it.

src/sdl/utils.cpp

@@ -1628,7 +1628,6 @@ surface blur_alpha_surface(const surface &surf, int depth)
 
 		p = lock.pixels() + y*res->w;
 		for(x = 0; x < res->w; ++x, ++p) {
-			assert(avg);
 			*p = (std::min(alpha/avg,ff) << 24) | (std::min(red/avg,ff) << 16) | (std::min(green/avg,ff) << 8) | std::min(blue/avg,ff);
 			if(x >= depth) {
 				alpha -= ((*front) >> 24)&0xFF;