renderergl2: reimplement greyscale as backend post after all 2D+3D draws

Fixes #654 - HUD/UI previously bypassed greyscale.
r_greyscale now supports partial desaturation [0-1].
Removed unused legacy logic from tr_shade.c

code/renderergl2/glsl/greyscale_fp.glsl

@@ -0,0 +1,15 @@
+uniform sampler2D u_TextureMap;
+uniform float     u_Greyscale;
+varying vec2      var_TexCoords;
+
+// From Rec. 709-1 4.2 "Derivation of luminance signal"
+const vec3 LUMA = vec3(0.2125, 0.7154, 0.0721);
+
+void main() {
+    vec4 color = texture2D(u_TextureMap, var_TexCoords);
+    if (u_Greyscale > 0.0) {
+        float y = dot(color.rgb, LUMA);
+        color.rgb = mix(color.rgb, vec3(y), clamp(u_Greyscale, 0.0, 1.0));
+    }
+    gl_FragColor = color;
+}

code/renderergl2/glsl/greyscale_vp.glsl

@@ -0,0 +1,11 @@
+attribute vec4 attr_Position;
+attribute vec2 attr_TexCoord0;
+varying   vec2 var_TexCoords;
+
+void main() {
+    vec2 clipXY = (attr_Position.xy * r_FBufScale) * 2.0 - 1.0;
+    clipXY.y = -clipXY.y;
+
+    gl_Position   = vec4(clipXY, 0.0, 1.0);
+    var_TexCoords = attr_TexCoord0;
+}