Increase fill_extrusion base/height precision.

Fixes issue #7247: heights over 65,536 meters don't render on some devices.
See discussion in issue #2096.

src/shaders/fill_extrusion.fragment.glsl

@@ -1,13 +1,6 @@
 varying vec4 v_color;
-#pragma mapbox: define lowp float base
-#pragma mapbox: define lowp float height
-#pragma mapbox: define highp vec4 color
 
 void main() {
-    #pragma mapbox: initialize lowp float base
-    #pragma mapbox: initialize lowp float height
-    #pragma mapbox: initialize highp vec4 color
-
     gl_FragColor = v_color;
 
 #ifdef OVERDRAW_INSPECTOR

src/shaders/fill_extrusion.vertex.glsl

@@ -8,14 +8,14 @@ attribute vec4 a_normal_ed;
 
 varying vec4 v_color;
 
-#pragma mapbox: define lowp float base
-#pragma mapbox: define lowp float height
+#pragma mapbox: define highp float base
+#pragma mapbox: define highp float height
 
 #pragma mapbox: define highp vec4 color
 
 void main() {
-    #pragma mapbox: initialize lowp float base
-    #pragma mapbox: initialize lowp float height
+    #pragma mapbox: initialize highp float base
+    #pragma mapbox: initialize highp float height
     #pragma mapbox: initialize highp vec4 color
 
     vec3 normal = a_normal_ed.xyz;