bmx library: set bpp header field correctly on save

compiler/res/prog8lib/cx16/bmx.p8

@@ -170,6 +170,7 @@ save_end:
     sub set_bpp(ubyte bpp) {
         ubyte[8] depths = [0,1,1,2,2,2,2,3]
         vera_colordepth = depths[bpp-1]
+        bitsperpixel = bpp
     }
 
     sub set_vera_colordepth(ubyte depth) {

docs/source/todo.rst

@@ -1,6 +1,8 @@
 TODO
 ====
 
+- add more optimized routine in conv to convert (u)byte to string.  (it now reuses the pretty large word to string routine)
+
 ...