grid render a few cells

shaders/cell.f.glsl

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+#version 330 core
+
+/// The background color for this cell.
+flat in vec4 bg_color;
+
+void main() {
+    gl_FragColor = bg_color;
+}

shaders/cell.v.glsl

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+#version 330 core
+
+// The grid coordinates (x, y) where x < columns and y < rows
+layout (location = 0) in vec2 grid_coord;
+
+// The background color for this cell in RGBA (0 to 1.0)
+layout (location = 1) in vec4 bg_color_in;
+
+// The background color for this cell in RGBA (0 to 1.0)
+flat out vec4 bg_color;
+
+uniform vec2 cell_dims;
+uniform mat4 projection;
+
+void main() {
+    // Top-left cell coordinates converted to world space
+    vec2 cell_pos = cell_dims * grid_coord;
+
+    // Turn the cell position into a vertex point depending on the
+    // gl_VertexID. Since we use instanced drawing, we have 4 vertices
+    // for each corner of the cell. We can use gl_VertexID to determine
+    // which one we're looking at. Using this, we can use 1 or 0 to keep
+    // or discard the value for the vertex.
+    vec2 position;
+    position.x = (gl_VertexID == 0 || gl_VertexID == 1) ? 1. : 0.;
+    position.y = (gl_VertexID == 0 || gl_VertexID == 3) ? 0. : 1.;
+    cell_pos = cell_pos + cell_dims * position;
+
+    gl_Position = projection * vec4(cell_pos, 1.0, 1.0);
+    bg_color = bg_color_in;
+}