[core] Double maximum label width by reducing glyph precision.

Fixes issue #11507.
Port of GL JS PR #6375.

Author: ChrisLoer

mapbox-gl-js

@@ -61,8 +61,8 @@ struct SymbolLayoutAttributes : gl::Attributes<
             {{
                 static_cast<int16_t>(labelAnchor.x),
                 static_cast<int16_t>(labelAnchor.y),
-                static_cast<int16_t>(::round(o.x * 64)),  // use 1/64 pixels for placement
-                static_cast<int16_t>(::round((o.y + glyphOffsetY) * 64))
+                static_cast<int16_t>(::round(o.x * 32)),  // use 1/32 pixels for placement
+                static_cast<int16_t>(::round((o.y + glyphOffsetY) * 32))
             }},
             {{
                 tx,

src/mbgl/programs/symbol_program.hpp

@@ -105,7 +105,7 @@ void main() {
     mat2 rotation_matrix = mat2(angle_cos, -1.0 * angle_sin, angle_sin, angle_cos);
 
     vec4 projected_pos = u_label_plane_matrix * vec4(a_projected_pos.xy, 0.0, 1.0);
-    gl_Position = u_gl_coord_matrix * vec4(projected_pos.xy / projected_pos.w + rotation_matrix * (a_offset / 64.0 * fontScale), 0.0, 1.0);
+    gl_Position = u_gl_coord_matrix * vec4(projected_pos.xy / projected_pos.w + rotation_matrix * (a_offset / 32.0 * fontScale), 0.0, 1.0);
 
     v_tex = a_tex / u_texsize;
     vec2 fade_opacity = unpack_opacity(a_fade_opacity);

src/mbgl/shaders/symbol_icon.cpp

@@ -183,7 +183,7 @@ void main() {
     mat2 rotation_matrix = mat2(angle_cos, -1.0 * angle_sin, angle_sin, angle_cos);
 
     vec4 projected_pos = u_label_plane_matrix * vec4(a_projected_pos.xy, 0.0, 1.0);
-    gl_Position = u_gl_coord_matrix * vec4(projected_pos.xy / projected_pos.w + rotation_matrix * (a_offset / 64.0 * fontScale), 0.0, 1.0);
+    gl_Position = u_gl_coord_matrix * vec4(projected_pos.xy / projected_pos.w + rotation_matrix * (a_offset / 32.0 * fontScale), 0.0, 1.0);
     float gamma_scale = gl_Position.w;
 
     vec2 tex = a_tex / u_texsize;