Add a visualisation mode for depth sprites.

For now this displays the raw sprites.  It will
eventually be repurposed for rendering the proper
depth data.

glsl/shp.frag

@@ -1,5 +1,7 @@
 uniform sampler2D DiffuseTexture, Palette;
 
+uniform bool EnableDepthPreview;
+
 varying vec4 TexCoord;
 varying vec4 ChannelMask;
 varying vec4 DepthMask;
@@ -8,5 +10,17 @@ void main()
 {
 	vec4 x = texture2D(DiffuseTexture, TexCoord.st);
 	vec2 p = vec2(dot(x, ChannelMask), TexCoord.p);
-	gl_FragColor = texture2D(Palette, p);
+	vec4 c = texture2D(Palette, p);
+
+	// Discard any transparent fragments (both color and depth)
+	if (c.a == 0.0)
+		discard;
+
+	if (EnableDepthPreview && length(DepthMask) > 0.0)
+	{
+		float depth = dot(x, DepthMask);
+		gl_FragColor = vec4(depth, depth, depth, 1);
+	}
+	else
+		gl_FragColor = c;
 }