cogl: Don't use GL_ALPHA

It was deprecated in OpenGL and it isn't even a valid value anymore for
functions like `glTextImage2D()` in OpenGL 4.

https://gitlab.gnome.org/GNOME/mutter/merge_requests/612

cogl/cogl/driver/gl/gl/cogl-driver-gl.c

@@ -111,20 +111,8 @@ _cogl_driver_pixel_format_to_gl (CoglContext     *context,
   switch (format)
     {
     case COGL_PIXEL_FORMAT_A_8:
-      /* If the driver doesn't natively support alpha textures then we
-       * will use a red component texture with a swizzle to implement
-       * the texture */
-      if (_cogl_has_private_feature
-          (context, COGL_PRIVATE_FEATURE_ALPHA_TEXTURES) == 0)
-        {
-          glintformat = GL_RED;
-          glformat = GL_RED;
-        }
-      else
-        {
-          glintformat = GL_ALPHA;
-          glformat = GL_ALPHA;
-        }
+      glintformat = GL_RED;
+      glformat = GL_RED;
       gltype = GL_UNSIGNED_BYTE;
       break;
     case COGL_PIXEL_FORMAT_G_8:

cogl/cogl/driver/gl/gl/cogl-texture-driver-gl.c

@@ -98,11 +98,8 @@ _cogl_texture_driver_gen (CoglContext *ctx,
       g_assert_not_reached();
     }
 
-  /* If the driver doesn't support alpha textures directly then we'll
-   * fake them by setting the swizzle parameters */
-  if (internal_format == COGL_PIXEL_FORMAT_A_8 &&
-      !_cogl_has_private_feature (ctx, COGL_PRIVATE_FEATURE_ALPHA_TEXTURES) &&
-      _cogl_has_private_feature (ctx, COGL_PRIVATE_FEATURE_TEXTURE_SWIZZLE))
+  /* GL_ALPHA is deprecated; instead, use GL_RED with a swizzle mask */
+  if (internal_format == COGL_PIXEL_FORMAT_A_8)
     {
       static const GLint red_swizzle[] = { GL_ZERO, GL_ZERO, GL_ZERO, GL_RED };
 

cogl/cogl/driver/gl/gles/cogl-driver-gles.c

@@ -83,8 +83,8 @@ _cogl_driver_pixel_format_to_gl (CoglContext     *context,
   switch (format)
     {
     case COGL_PIXEL_FORMAT_A_8:
-      glintformat = GL_ALPHA;
-      glformat = GL_ALPHA;
+      glintformat = GL_RED;
+      glformat = GL_RED;
       gltype = GL_UNSIGNED_BYTE;
       break;
     case COGL_PIXEL_FORMAT_G_8: