[examples] Added an example for raylib OpenGL interop (#1726)

* Added an example for raylib OpenGL interop.

* Removed C99 variable-length array to fix MSVC errors

* Moved the opengl interop example from shaders to others.

examples/others/resources/shaders/glsl330/point_particle.fs

@@ -0,0 +1,17 @@
+#version 330
+
+// Input uniform values
+uniform vec4 color;
+
+// Output fragment color
+out vec4 finalColor;
+
+// NOTE: Add here your custom variables
+
+void main()
+{
+    // Each point is drawn as a screen space square of gl_PointSize size. gl_PointCoord contains where we are inside of
+    // it. (0, 0) is the top left, (1, 1) the bottom right corner.
+    // Draw each point as a colored circle with alpha 1.0 in the center and 0.0 at the outer edges.
+    finalColor = vec4(color.rgb, color.a * (1 - length(gl_PointCoord.xy - vec2(0.5))*2));
+}

examples/others/resources/shaders/glsl330/point_particle.vs

@@ -0,0 +1,24 @@
+#version 330
+
+// Input vertex attributes
+in vec3 vertexPosition;
+
+// Input uniform values
+uniform mat4 mvp;
+uniform float currentTime;
+
+// NOTE: Add here your custom variables 
+
+void main()
+{
+    // Unpack data from vertexPosition
+    vec2  pos    = vertexPosition.xy;
+    float period = vertexPosition.z;
+    
+    // Calculate final vertex position (jiggle it around a bit horizontally)
+    pos += vec2(100, 0) * sin(period * currentTime);
+    gl_Position = mvp * vec4(pos, 0.0, 1.0);
+    
+    // Calculate the screen space size of this particle (also vary it over time)
+    gl_PointSize = 10 - 5 * abs(sin(period * currentTime));
+}