common: generate atlas in y rather than x direction

[mewmew]

This lets us generate atlases that are more "square"ish,
and thus less likely to hit the graphics card boundaries
for texture dimensions.

Fixes #121.

[mewmew]

Prior to this PR, there were atlases that required a width larger than 8192. After this PR, no atlas requires more than 8192 in either width nor height.

Current set of atlases generated:

create atlas
   totalWidth: 32
   totalHeight: 214
create atlas
   totalWidth: 256
   totalHeight: 2560
create atlas
   totalWidth: 35
   totalHeight: 7680
create atlas
   totalWidth: 15
   totalHeight: 3840
create atlas
   totalWidth: 256
   totalHeight: 2400
create atlas
   totalWidth: 256
   totalHeight: 2400
create atlas
   totalWidth: 178
   totalHeight: 4676
create atlas
   totalWidth: 184
   totalHeight: 4505
create atlas
   totalWidth: 159
   totalHeight: 3660
create atlas
   totalWidth: 159
   totalHeight: 3660
create atlas
   totalWidth: 17
   totalHeight: 4608
create atlas
   totalWidth: 256
   totalHeight: 140
create atlas
   totalWidth: 12
   totalHeight: 3584
create atlas
   totalWidth: 135
   totalHeight: 50
create atlas
   totalWidth: 135
   totalHeight: 50
create atlas
   totalWidth: 256
   totalHeight: 140
create atlas
   totalWidth: 256
   totalHeight: 140
create atlas
   totalWidth: 256
   totalHeight: 140

[essial]

Thank you so much!