mesa: move glGenerateMipmap() code into new genmipmap.c file

Mipmap generation has nothing to do with FBOs.
v2: update gl_genexec.py too (not api_exec.c)

Acked-by: Kenneth Graunke <kenneth@whitecape.org>

src/mesa/SConscript

@@ -78,6 +78,7 @@ main_sources = [
     'main/format_pack.c',
     'main/format_unpack.c',
     'main/framebuffer.c',
+    'main/genmipmap.c',
     'main/getstring.c',
     'main/glformats.c',
     'main/hash.c',