[vulkan] Get spotlight shadows working

They currently have a hard-coded bias of 0.5 pixels (if I'm doing my
math correctly) to combat the shadow acne, but look pretty good.

libs/video/renderer/vulkan/rp_main_def.plist

@@ -1555,7 +1555,7 @@ renderpasses = {
 					color = {
 						light = {
 							layout = color_attachment_optimal;
-							blend = $blend_disable;
+							blend = $additive_blend;
 						};
 					};
 					preserve = (depth, output);

libs/video/renderer/vulkan/shader/lighting_plane.frag

@@ -1,12 +1,19 @@
 #version 450
 #extension GL_GOOGLE_include_directive : enable
 
+layout (set = 0, binding = 0) buffer ShadowMatrices {
+	mat4 shadow_mats[];
+};
 layout (set = 3, binding = 0) uniform sampler2DArrayShadow shadow_map[32];
 
 float
 shadow (uint map_id, uint layer, uint mat_id, vec3 pos)
 {
-	return 1;
+	vec4 p = shadow_mats[mat_id] * vec4 (pos, 1);
+	p = p / (p.w - 0.5);	//FIXME hard-coded bias
+	float depth = p.z;
+	vec2 uv = (p.xy + vec2(1)) / 2;
+	return texture (shadow_map[map_id], vec4 (uv, layer, depth));
 }
 
 #include "lighting_main.finc"

libs/video/renderer/vulkan/smp_quake.plist

@@ -112,7 +112,7 @@ samplers = {
 		anisotropyEnable = false;
 		maxAnisotropy = 0;
 		compareEnable = true;
-		compareOp = less;
+		compareOp = greater_or_equal;
 		minLod = 0;
 		maxLod = 1000;
 		borderColor = float_opaque_white;