Add skeleton of TextRenderer

Signed-off-by: Slendi <slendi@socopon.com>

src/msdf.fs

@@ -0,0 +1,34 @@
+#version 100
+#extension GL_OES_standard_derivatives : enable
+
+#ifdef GL_ES
+precision mediump float;
+#endif
+
+varying vec2 fragTexCoord;
+varying vec4 fragColor;
+
+uniform sampler2D texture0;
+uniform vec4 colDiffuse;
+uniform float pxRange;
+
+float median(float r, float g, float b) {
+	return max(min(r, g), min(max(r, g), b));
+}
+
+float screenPxRange(vec2 uv) {
+	vec2 duv_dx = dFdx(uv);
+	vec2 duv_dy = dFdy(uv);
+	float scale = 0.5 * (length(duv_dx) + length(duv_dy));
+	return max(scale * pxRange, 1.0);
+}
+
+void main() {
+	vec3 msd = texture2D(texture0, fragTexCoord).rgb;
+	float sd = median(msd.r, msd.g, msd.b);
+	float spx = screenPxRange(fragTexCoord);
+	float dist = spx * (sd - 0.5);
+	float opacity = clamp(dist + 0.5, 0.0, 1.0);
+
+	gl_FragColor = vec4(fragColor.rgb, fragColor.a * opacity) * colDiffuse;
+}