tracing – How to place a ListPlot in a Texture without losing resolution?

I have a code that produces the left plot (vectorized image) and makes it a Texture in order to have it under a ListPlot3D (right):

enter the description of the image here

Problem: The left image loses its resolution when it is transformed into Texture.

Here is a MWE:

a = ListPlot[Table[{i, j}, {i, 1, 13, 1}, {j, 1, 13, 1}],
Plot Style ->
directive[Opacity[0.5], Darker[Blue], PointSize[0.025]],
AspectRatio -> 1];
max = 6; level = 0
EdgeForm[]{Polygon[{{-maxg, -maxg, level}, {maxg, -maxg, 
      level}, {maxg, maxg, level}, {-maxg, maxg, level}}, 
    VertexTextureCoordinates -> {{0, 0}, {1, 0}, {1, 1}, {0, 1}}]}}
Lighting -> {{"Ambient", White}}]

How can I make sure that the plot in my Graphics3D is still vectorized?