add basic imperative shell

shell/gl-shaders.sml

@@ -0,0 +1,23 @@
+structure GlShaders =
+struct
+  val xyrgbVertexShaderString =
+    "#version 300 es\n\
+    \layout (location = 0) in vec2 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\
+    \}"
+
+  val rgbFragmentShaderString =
+    "#version 300 es\n\
+    \precision mediump float;\n\
+    \in vec3 frag_col;\n\
+    \out vec4 FragColor;\n\
+    \void main()\n\
+    \{\n\
+    \   FragColor = vec4(frag_col.x, frag_col.y, frag_col.z, 1.0f);\n\
+    \}"
+end