add mix shader for ES3

TODO: translate shader

data/ofxMsdfgen/shaders/mix/ES3/shader.frag

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+#version 300 es
+precision highp float;
+
+// these are our textures
+uniform sampler2D msdf;
+
+uniform vec4 fontColor;
+uniform vec2 unitRange;
+uniform float msdf_mix;
+uniform vec2 translation_a;
+uniform vec2 scale_a;
+uniform vec2 translation_b;
+uniform vec2 scale_b;
+
+// this comes from the vertex shader
+in vec2 texCoordVarying;
+out vec4 outputColor;
+
+float median(float r, float g, float b) {
+    return max(min(r, g), min(max(r, g), b));
+}
+
+float screenPxRange() {
+    //ivec2 ts = textureSize(msdf, 0);
+    //vec2 unitRange = vec2(pxRange)/vec2(ts);
+    vec2 screenTexSize = vec2(1.0)/fwidth(texCoordVarying);
+    return max(0.5*dot(unitRange, screenTexSize), 1.0);
+}
+
+float screenPxDistance(vec3 msdf_rgb) {
+    float sd = median(msdf_rgb.r, msdf_rgb.g, msdf_rgb.b);
+    return screenPxRange()*(sd - 0.5);
+}
+
+void main()
+{
+    vec4 bgColor = vec4(fontColor.rgb,0.0);
+    vec4 fgColor = fontColor;
+
+    vec2 texCoord_a = (texCoordVarying * scale_a) + translation_a;
+    vec2 texCoord_b = (texCoordVarying * scale_b) + translation_b;
+
+    vec3 msdf_rgb_total = texture(msdf, texCoordVarying).rgb;
+    vec3 msdf_rgb_a = texture(msdf, texCoord_a).rgb;
+    vec3 msdf_rgb_b = texture(msdf, texCoord_b).rgb;
+    float screenPxDistance_a = screenPxDistance(msdf_rgb_a);
+    float screenPxDistance_b = screenPxDistance(msdf_rgb_b);
+    float screenPxDistance_mix = mix(screenPxDistance_a, screenPxDistance_b, msdf_mix);
+
+    float opacity = clamp(screenPxDistance_mix + 0.5, 0.0, 1.0);
+
+    if (opacity < 0.5)
+        discard;
+    outputColor = vec4(vec3(fgColor),1.0); //mix(bgColor, fgColor, opacity);
+
+    //outputColor = vec4(opacity, 1.0-opacity, 0.0, 1.0);
+    //outputColor = vec4(1.0-msdf_rgb.rgb, 1.0);
+    //outputColor = vec4(1.0,0.0,1.0,1.0);
+}

data/ofxMsdfgen/shaders/mix/ES3/shader.vert

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+#version 300 es
+
+// whoops these are for the programmable pipeline system
+uniform mat4 modelViewProjectionMatrix;
+
+in vec4 position;
+in vec2 texcoord;
+
+out vec2 texCoordVarying;
+
+void main()
+{
+    texCoordVarying = texcoord;
+    gl_Position = modelViewProjectionMatrix * position;
+}