simplify shader grid drawing and add option to use a texture for the grid

shaders/board_f.glsl

@@ -1,6 +1,10 @@
 #version 330 core
 out vec4 FragColor;
 
+in vec2 texCoords;
+
+uniform sampler2D aTexture;
+
 void main() {
-  FragColor = vec4(240.f/255.f, 235.f/255.f, 227.f/255.f, 1.0);
+  FragColor = texture(aTexture, texCoords);
 }