begin refactoring code to use a single vertex buffer for all shapes

shell/gl-shaders.sml

@@ -2,13 +2,13 @@ structure GlShaders =
 struct
   val xyrgbVertexShaderString =
     "#version 300 es\n\
-    \layout (location = 0) in vec2 apos;\n\
+    \layout (location = 0) in vec3 apos;\n\
     \layout (location = 1) in vec3 col;\n\
     \out vec3 frag_col;\n\
     \void main()\n\
     \{\n\
     \   frag_col = col;\n\
-    \   gl_Position = vec4(apos.x, apos.y, 0.0f, 1.0f);\n\
+    \   gl_Position = vec4(apos.x, apos.y, apos.z, 1.0f);\n\
     \}"
 
   val rgbFragmentShaderString =