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_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;
+}