Reduce precision of floats in fragment shader

~0.43ms/frame -> ~0.37ms/frame

src/renderer/webgl/GlyphRenderer.ts

@@ -55,7 +55,7 @@ void main() {
 }`;
 
 const fragmentShaderSource = `#version 300 es
-precision mediump float;
+precision lowp float;
 
 in vec2 v_texcoord;
 

src/renderer/webgl/RectangleRenderer.ts

@@ -36,7 +36,7 @@ void main() {
 }`;
 
 const fragmentShaderSource = `#version 300 es
-precision mediump float;
+precision lowp float;
 
 in vec3 v_color;