fragment shader from glamor:

precision mediump float;

varying vec2 glyph_pos;
uniform sampler2D sampler;
uniform sampler2D atlas;
void main() {
       vec4 mask = texture2D(atlas, glyph_pos);
       vec4 source = texture2D(sampler, vec2(0.5));
       gl_FragColor = source * mask.a;
}

Signed-off-by: Qiang Yu <yuq825@gmail.com>