never scale images when copying them into the SpriteAtlas

[jfirebaugh]

Currently we bilinearly scale images that are not at the native pixel ratio up or down when adding them to the SpriteAtlas. This has led to code that is more complex than necessary and produced issues like #3031 and #2198.

Instead, we should copy images into the SpriteAtlas pixel-for-pixel, and let up/down sampling happen in the shaders.

@kkaefer @ansis -- am I missing anything that would make this non-viable?

[ansis]

Instead, we should copy images into the SpriteAtlas pixel-for-pixel, and let up/down sampling happen in the shaders.
@kkaefer @ansis -- am I missing anything that would make this non-viable?

I agree that we should do it this way. This is what we're still doing in -js. The scaling was never ported. relevant discussion: mapbox/mapbox-gl-js#1124

[jfirebaugh]

@kkaefer @ansis Is there any significance to the use of 2 here:

https://github.com/mapbox/mapbox-gl-js/blob/abe3512815032f02a0edefe38c732b4431535b3a/js/symbol/sprite_atlas.js#L114

While 1 is used everywhere else:

https://github.com/mapbox/mapbox-gl-js/blob/abe3512815032f02a0edefe38c732b4431535b3a/js/symbol/sprite_atlas.js#L91
https://github.com/mapbox/mapbox-gl-js/blob/abe3512815032f02a0edefe38c732b4431535b3a/js/symbol/sprite_atlas.js#L173

mapbox-gl-native/src/mbgl/sprite/sprite_atlas.cpp

Line 99 in 2a0d2b7

const float padding = 1;

mapbox-gl-native/src/mbgl/sprite/sprite_atlas.cpp

Line 120 in 2a0d2b7

const int offset = 1;

mapbox-gl-native/src/mbgl/sprite/sprite_atlas.cpp

Line 134 in 2a0d2b7

const uint32_t border = 1;

If not, I'm going to use 1 everywhere and remove the duplication.

[ansis]

I'm not sure, but I think the 2 means 1px padding on each of the two sides. I think it's correct but not clear.

[jfirebaugh]

native definitely uses 1 at the equivalent spot.

[jfirebaugh]

This is getting bumped from my priorities.