demo: Add GLES support to the GtkGLArea demo

We need new shaders, and we need to select the correct shader when
building the program we use to render the triangle.

demos/gtk-demo/demo.gresource.xml

@@ -199,8 +199,10 @@
     <file>popover.ui</file>
   </gresource>
   <gresource prefix="/glarea">
-    <file>glarea-fragment.glsl</file>
-    <file>glarea-vertex.glsl</file>
+    <file>glarea-gl.fs.glsl</file>
+    <file>glarea-gl.vs.glsl</file>
+    <file>glarea-gles.fs.glsl</file>
+    <file>glarea-gles.vs.glsl</file>
   </gresource>
   <gresource prefix="/font_features">
     <file>font-features.ui</file>

demos/gtk-demo/glarea-gles.fs.glsl

@@ -0,0 +1,7 @@
+precision highp float;
+
+void main() {
+  float lerpVal = gl_FragCoord.y / 500.0f;
+
+  gl_FragColor = mix(vec4(1.0f, 0.85f, 0.35f, 1.0f), vec4(0.2f, 0.2f, 0.2f, 1.0f), lerpVal);
+}

demos/gtk-demo/glarea-gles.vs.glsl

@@ -0,0 +1,7 @@
+attribute vec4 position;
+
+uniform mat4 mvp;
+
+void main() {
+  gl_Position = mvp * position;
+}

demos/gtk-demo/glarea.c

@@ -93,7 +93,9 @@ create_shader (int         type,
 
 /* Initialize the shaders and link them into a program */
 static void
-init_shaders (GLuint *program_out,
+init_shaders (const char *vertex_path,
+              const char *fragment_path,
+              GLuint *program_out,
               GLuint *mvp_out)
 {
   GLuint vertex, fragment;
@@ -102,7 +104,7 @@ init_shaders (GLuint *program_out,
   int status;
   GBytes *source;
 
-  source = g_resources_lookup_data ("/glarea/glarea-vertex.glsl", 0, NULL);
+  source = g_resources_lookup_data (vertex_path, 0, NULL);
   vertex = create_shader (GL_VERTEX_SHADER, g_bytes_get_data (source, NULL));
   g_bytes_unref (source);
 
@@ -112,7 +114,7 @@ init_shaders (GLuint *program_out,
       return;
     }
 
-  source = g_resources_lookup_data ("/glarea/glarea-fragment.glsl", 0, NULL);
+  source = g_resources_lookup_data (fragment_path, 0, NULL);
   fragment = create_shader (GL_FRAGMENT_SHADER, g_bytes_get_data (source, NULL));
   g_bytes_unref (source);
 
@@ -218,13 +220,29 @@ static GLuint mvp_location;
 static void
 realize (GtkWidget *widget)
 {
+  const char *vertex_path, *fragment_path;
+  GdkGLContext *context;
+
   gtk_gl_area_make_current (GTK_GL_AREA (widget));
 
   if (gtk_gl_area_get_error (GTK_GL_AREA (widget)) != NULL)
     return;
 
+  context = gtk_gl_area_get_context (GTK_GL_AREA (widget));
+
+  if (gdk_gl_context_get_use_es (context))
+    {
+      vertex_path = "/glarea/glarea-gles.vs.glsl";
+      fragment_path = "/glarea/glarea-gles.fs.glsl";
+    }
+  else
+    {
+      vertex_path = "/glarea/glarea-gl.vs.glsl";
+      fragment_path = "/glarea/glarea-gl.fs.glsl";
+    }
+
   init_buffers (&position_buffer, NULL);
-  init_shaders (&program, &mvp_location);
+  init_shaders (vertex_path, fragment_path, &program, &mvp_location);
 }
 
 /* We should tear down the state when unrealizing */