Fix precision issue in gradient skybox shader

[mpulkki-mapbox]

Gradient skybox is suffering from precision issues on some of the ARM Mali gpus resulting in "wobbly"/jittery movement of the gradient effect on both gl-js and gl-native. This is caused by the uniform u_center_direction in skybox_gradient.fragment.glsl shader being defined as lowp. Increasing the precision to highp fixes the issue.

Record_2021_0525_1300-00.00.18.531-00.00.36.515.mp4

• briefly describe the changes in this PR
• include before/after visuals or gifs if this PR includes visual changes
• manually test the debug page
• apply changelog label ('bug', 'feature', 'docs', etc) or use the label 'skip changelog'
• add an entry inside this element for inclusion in the mapbox-gl-js changelog: <changelog>Fix gradient skybox rendering issue on some ARM Mali gpus</changelog>