[core] Fix aliased text on mobile GPUs

src/mbgl/shaders/symbol_sdf.cpp

@@ -168,7 +168,7 @@ uniform sampler2D u_fadetexture;
 uniform lowp vec4 u_color;
 uniform lowp float u_opacity;
 uniform lowp float u_buffer;
-uniform lowp float u_gamma;
+uniform highp float u_gamma;
 
 varying vec2 v_tex;
 varying vec2 v_fade_tex;
@@ -177,8 +177,8 @@ varying float v_gamma_scale;
 void main() {
     lowp float dist = texture2D(u_texture, v_tex).a;
     lowp float fade_alpha = texture2D(u_fadetexture, v_fade_tex).a;
-    lowp float gamma = u_gamma * v_gamma_scale;
-    lowp float alpha = smoothstep(u_buffer - gamma, u_buffer + gamma, dist) * fade_alpha;
+    highp float gamma = u_gamma * v_gamma_scale;
+    highp float alpha = smoothstep(u_buffer - gamma, u_buffer + gamma, dist) * fade_alpha;
 
     gl_FragColor = u_color * (alpha * u_opacity);