Review VR simulator mechanism

- No default VR device parameteres inside raylib
- VR device parameter should be provided by user
- VR distortion shader should be provided by user

examples/core/resources/distortion.fs

@@ -0,0 +1,43 @@
+#version 330
+
+in vec2 fragTexCoord;
+in vec4 fragColor;
+out vec4 finalColor;
+
+uniform sampler2D texture0;
+uniform vec2 leftLensCenter = vec2(0.288, 0.5);
+uniform vec2 rightLensCenter = vec2(0.712, 0.5);
+uniform vec2 leftScreenCenter = vec2(0.25, 0.5);
+uniform vec2 rightScreenCenter = vec2(0.75, 0.5);
+uniform vec2 scale = vec2(0.25, 0.45);
+uniform vec2 scaleIn = vec2(4, 2.2222);
+uniform vec4 hmdWarpParam = vec4(1, 0.22, 0.24, 0);
+uniform vec4 chromaAbParam = vec4(0.996, -0.004, 1.014, 0.0);
+
+void main()
+{
+    vec2 lensCenter = fragTexCoord.x < 0.5? leftLensCenter : rightLensCenter;
+    vec2 screenCenter = fragTexCoord.x < 0.5? leftScreenCenter : rightScreenCenter;
+    vec2 theta = (fragTexCoord - lensCenter)*scaleIn;
+    float rSq = theta.x*theta.x + theta.y*theta.y;
+    vec2 theta1 = theta*(hmdWarpParam.x + hmdWarpParam.y*rSq + hmdWarpParam.z*rSq*rSq + hmdWarpParam.w*rSq*rSq*rSq);
+    vec2 thetaBlue = theta1*(chromaAbParam.z + chromaAbParam.w*rSq);
+    vec2 tcBlue = lensCenter + scale*thetaBlue;
+
+    if (any(bvec2(clamp(tcBlue, screenCenter - vec2(0.25, 0.5), screenCenter + vec2(0.25, 0.5)) - tcBlue)))
+    {
+        finalColor = vec4(0.0, 0.0, 0.0, 1.0);
+    }
+    else
+    {
+        float blue = texture(texture0, tcBlue).b;
+        vec2 tcGreen = lensCenter + scale*theta1;
+        float green = texture(texture0, tcGreen).g;
+
+        vec2 thetaRed = theta1*(chromaAbParam.x + chromaAbParam.y*rSq);
+        vec2 tcRed = lensCenter + scale*thetaRed;
+
+        float red = texture(texture0, tcRed).r;
+        finalColor = vec4(red, green, blue, 1.0);
+    }
+};