Align vertex attributes at 4 byte boundaries

iOS devices [render a lot faster](https://developer.apple.com/library/ios/documentation/3DDrawing/Conceptual/OpenGLES_ProgrammingGuide/TechniquesforWorkingwithVertexData/TechniquesforWorkingwithVertexData.html#//apple_ref/doc/uid/TP40008793-CH107-SW7) when all shader attributes are aligned at 4 byte boundaries. This pull request changes the SDF/Icon/Line shader to be aligned. It does so by reverting 9d5f02ccaa051a0f3459f6fc94df807338fa2552 and 4c0b4a79017bc7c7ae864e6f961f186605be617f, which reverted https://github.com/mapbox/mapbox-gl-native/commit/d5b4c1162dab706d6f56881e3f72baa003ad8ac7 and https://github.com/mapbox/mapbox-gl-native/commit/3a9a9043785ab0272808753d54036948b05a89b1.

This changes the shader so that they are no longer the same as in the JS version. We should port this change to JS, since it is likely that it will also benefit from the aligned data access.

/cc @jfirebaugh