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

shaders/board_v.glsl

@@ -1,8 +1,11 @@
 #version 330 core
 layout (location = 0) in vec3 aPos;
+layout (location = 1) in vec2 aTexCoords;
+out vec2 texCoords;
 
 uniform mat4 transform;
 
 void main() {
   gl_Position = transform * vec4(aPos, 1.0f);
+  texCoords = aTexCoords;
 }