[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));
+}