add more model & shader examples

examples/shaders/eratosthenes/eratosthenes.fs

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+#version 330
+
+/*************************************************************************************
+
+  The Sieve of Eratosthenes -- a simple shader by ProfJski
+  An early prime number sieve: https://en.wikipedia.org/wiki/Sieve_of_Eratosthenes
+
+  The screen is divided into a square grid of boxes, each representing an integer value.
+  Each integer is tested to see if it is a prime number.  Primes are colored white.
+  Non-primes are colored with a color that indicates the smallest factor which evenly divdes our integer.
+
+  You can change the scale variable to make a larger or smaller grid.
+  Total number of integers displayed = scale squared, so scale = 100 tests the first 10,000 integers.
+
+  WARNING: If you make scale too large, your GPU may bog down!
+
+***************************************************************************************/
+
+// Input vertex attributes (from vertex shader)
+in vec2 fragTexCoord;
+in vec4 fragColor;
+
+// Output fragment color
+out vec4 finalColor;
+
+// Make a nice spectrum of colors based on counter and maxSize
+vec4 Colorizer(float counter, float maxSize)
+{
+    float red = 0.0, green = 0.0, blue = 0.0;
+    float normsize = counter/maxSize;
+
+    red = smoothstep(0.3, 0.7, normsize);
+    green = sin(3.14159*normsize);
+    blue = 1.0 - smoothstep(0.0, 0.4, normsize);
+
+    return vec4(0.8*red, 0.8*green, 0.8*blue, 1.0);
+}
+
+void main()
+{
+    vec4 color = vec4(1.0);
+    float scale = 1000.0; // Makes 100x100 square grid. Change this variable to make a smaller or larger grid.
+    int value = int(scale*floor(fragTexCoord.y*scale)+floor(fragTexCoord.x*scale));  // Group pixels into boxes representing integer values
+
+    if ((value == 0) || (value == 1) || (value == 2)) finalColor = vec4(1.0);
+    else
+    {
+        for (int i = 2; (i < max(2, sqrt(value) + 1)); i++)
+        {
+            if ((value - i*floor(float(value)/float(i))) == 0)
+            {
+                color = Colorizer(float(i), scale);
+                //break;    // Uncomment to color by the largest factor instead
+            }
+        }
+
+        finalColor = color;
+    }
+}

examples/shaders/eratosthenes/main.go

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+package main
+
+import (
+	rl "github.com/gen2brain/raylib-go/raylib"
+)
+
+func main() {
+	screenWidth := int32(800)
+	screenHeight := int32(450)
+
+	rl.InitWindow(screenWidth, screenHeight, "raylib [shaders] example - Eratosthenes")
+
+	shader := rl.LoadShader("", "eratosthenes.fs")
+	target := rl.LoadRenderTexture(screenWidth, screenHeight)
+
+	rl.SetTargetFPS(60)
+
+	for !rl.WindowShouldClose() {
+
+		rl.BeginTextureMode(target)
+		rl.ClearBackground(rl.Black)
+		rl.DrawRectangle(0, 0, int32(rl.GetScreenWidth()), int32(rl.GetScreenHeight()), rl.Black)
+		rl.EndTextureMode()
+
+		rl.BeginDrawing()
+
+		rl.ClearBackground(rl.RayWhite)
+
+		rl.BeginShaderMode(shader)
+
+		rl.DrawTextureRec(target.Texture, rl.NewRectangle(0, 0, float32(target.Texture.Width), float32(target.Texture.Height)), rl.Vector2Zero(), rl.White)
+
+		rl.EndShaderMode()
+
+		rl.EndDrawing()
+	}
+
+	rl.UnloadShader(shader)
+	rl.UnloadRenderTexture(target)
+
+	rl.CloseWindow()
+}