add shader strings which will allow us to make an xyrgba shader in OpenGL

shell/gl-shaders.sml

@@ -20,4 +20,26 @@ struct
     \{\n\
     \   FragColor = vec4(frag_col.x, frag_col.y, frag_col.z, 1.0f);\n\
     \}"
+
+  (* shader strings same as above, except they also define an alpha component *)
+  val xyrgbaVertexShaderString =
+    "#version 300 es\n\
+    \layout (location = 0) in vec2 apos;\n\
+    \layout (location = 1) in vec4 col;\n\
+    \out vec4 frag_col;\n\
+    \void main()\n\
+    \{\n\
+    \   frag_col = col;\n\
+    \   gl_Position = vec4(apos.x, apos.y, 0.0f, 1.0f);\n\
+    \}"
+
+  val rgbaFragmentShaderString =
+    "#version 300 es\n\
+    \precision mediump float;\n\
+    \in vec4 frag_col;\n\
+    \out vec4 FragColor;\n\
+    \void main()\n\
+    \{\n\
+    \   FragColor = frag_col;\n\
+    \}"
 end