Port shadow shaders to work with OpenGL3

Co-authored-by: sfan5 <sfan5@live.de>

client/shaders/shadow/pass1_trans/opengl_fragment.glsl

@@ -0,0 +1,48 @@
+uniform sampler2D ColorMapSampler;
+varying vec4 tPos;
+
+#ifdef GL_ES
+varying mediump vec2 varTexCoord;
+#else
+centroid varying vec2 varTexCoord;
+#endif
+
+#ifdef COLORED_SHADOWS
+varying vec3 varColor;
+
+// c_precision of 128 fits within 7 base-10 digits
+const float c_precision = 128.0;
+const float c_precisionp1 = c_precision + 1.0;
+
+float packColor(vec3 color)
+{
+	return floor(color.b * c_precision + 0.5)
+		+ floor(color.g * c_precision + 0.5) * c_precisionp1
+		+ floor(color.r * c_precision + 0.5) * c_precisionp1 * c_precisionp1;
+}
+
+const vec3 black = vec3(0.0);
+#endif
+
+void main()
+{
+	vec4 col = texture2D(ColorMapSampler, varTexCoord);
+#ifndef COLORED_SHADOWS
+	if (col.a < 0.5)
+		discard;
+#endif
+
+	float depth = 0.5 + tPos.z * 0.5;
+	// ToDo: Liso: Apply movement on waving plants
+	// depth in [0, 1] for texture
+
+	//col.rgb = col.a == 1.0 ? vec3(1.0) : col.rgb;
+#ifdef COLORED_SHADOWS
+	col.rgb *= varColor.rgb;
+	// premultiply color alpha (see-through side)
+	float packedColor = packColor(col.rgb * (1.0 - col.a));
+	gl_FragColor = vec4(depth, packedColor, 0.0,1.0);
+#else
+	gl_FragColor = vec4(depth, 0.0, 0.0, 1.0);
+#endif
+}