ref_gl: introduce gl2_shim based on vgl_shim for future limited core/…

…gles context support (SLOW, still needs ffp for matrix operations)

ref/gl/gl2_shim/fragment.glsl.inc

@@ -0,0 +1,53 @@
+R"(
+precision mediump float;
+#if ATTR_TEXCOORD0
+uniform sampler2D uTex0;
+#endif
+#if ATTR_TEXCOORD1
+uniform sampler2D uTex1;
+#endif
+#if FEAT_ALPHA_TEST
+uniform float uAlphaTest;
+#endif
+#if FEAT_FOG
+uniform vec4 uFog;
+#endif
+uniform vec4 uColor;
+#if ATTR_COLOR
+in vec4 vColor;
+#endif
+#if ATTR_TEXCOORD0
+in vec2 vTexCoord0;
+#endif
+#if ATTR_TEXCOORD1
+in vec2 vTexCoord1;
+#endif
+#if ATTR_NORMAL
+in vec2 vNormal;
+#endif
+out vec4 oFragColor;
+void main()
+{
+#if ATTR_COLOR
+   vec4 c = vColor;
+#else
+   vec4 c = uColor;
+#endif
+#if ATTR_TEXCOORD0
+    c = c * texture(uTex0, vTexCoord0);
+#endif
+#if ATTR_TEXCOORD1
+    c = c * texture(uTex1, vTexCoord1);
+#endif
+#if FEAT_ALPHA_TEST
+    if(c.a <= uAlphaTest)
+        discard;
+#endif
+#if FEAT_FOG
+    float fogDist = gl_FragCoord.z / gl_FragCoord.w;
+    float fogRate = clamp(exp(-uFog.w * fogDist), 0.f, 1.f);
+    c.rgb = mix(uFog.rgb, c.rgb, fogRate);
+#endif
+    oFragColor = c;
+}
+)"