Add fog to terrain example

Quest and WebGL platforms currently don't have working wireframe, and
without it the terrain features are not recognizable. Fog adds depth
perception cue.

examples/Environment/Terrain/main.lua

@@ -1,4 +1,24 @@
-local terrain
+local terrain, shader
+
+local shaderCode = {[[
+/* VERTEX shader */
+out vec4 fragmentView;
+
+vec4 position(mat4 projection, mat4 transform, vec4 vertex) {
+  fragmentView = projection * transform * vertex;
+  return fragmentView;
+} ]], [[ 
+/* FRAGMENT shader */
+#define PI 3.1415926538
+in vec4 fragmentView;
+uniform vec3 fogColor;
+
+vec4 color(vec4 graphicsColor, sampler2D image, vec2 uv)
+{
+  float fogAmount = atan(length(fragmentView) * 0.1) * 2.0 / PI;
+  vec4 color = vec4(mix(graphicsColor.rgb, fogColor, fogAmount), graphicsColor.a);
+  return color;
+}]]}
 
 local function grid(subdivisions)
   local size = 1 / math.floor(subdivisions or 1)
@@ -25,8 +45,11 @@ local function grid(subdivisions)
 end
 
 function lovr.load()
-  lovr.graphics.setBackgroundColor(0.208, 0.208, 0.275)
+  local skyColor = {0.208, 0.208, 0.275}
+  lovr.graphics.setBackgroundColor(skyColor)
   lovr.graphics.setLineWidth(5)
+  shader = lovr.graphics.newShader(unpack(shaderCode))
+  shader:send('fogColor', { lovr.math.gammaToLinear(unpack(skyColor)) })
   terrain = grid(100)
   local offset = lovr.math.noise(0, 0) -- ensure zero height at origin
   for vi = 1, terrain:getVertexCount() do
@@ -37,12 +60,13 @@ function lovr.load()
 end
 
 function lovr.draw()
+  lovr.graphics.setShader(shader)
   lovr.graphics.rotate(math.pi/2, 1, 0, 0)
   lovr.graphics.scale(100)
   lovr.graphics.setWireframe(false)
   lovr.graphics.setColor(0.565, 0.404, 0.463)
   terrain:draw()
   lovr.graphics.setWireframe(true)
-  lovr.graphics.setColor(0.388, 0.302, 0.412, 0.5)
+  lovr.graphics.setColor(0.388, 0.302, 0.412, 0.1)
   terrain:draw()
 end