Rename GLES2 driver to OpenGL to prepare for the upgrade to GLES3

- Use lowercase driver names for the `--rendering-driver`
  command line argument.

SConstruct

@@ -15,7 +15,7 @@ from collections import OrderedDict
 # Local
 import methods
 import glsl_builders
-import gles_builders
+import opengl_builders
 from platform_methods import run_in_subprocess
 
 # Scan possible build platforms
@@ -708,20 +708,11 @@ if selected_platform in platform_list:
     }
     env.Append(BUILDERS=GLSL_BUILDERS)
 
-    if not env["platform"] == "server":  # FIXME: detect GLES3
+    if not env["platform"] == "server":
         env.Append(
             BUILDERS={
-                "GLES3_GLSL": env.Builder(
-                    action=run_in_subprocess(gles_builders.build_gles3_headers),
-                    suffix="glsl.gen.h",
-                    src_suffix=".glsl",
-                )
-            }
-        )
-        env.Append(
-            BUILDERS={
-                "GLES2_GLSL": env.Builder(
-                    action=run_in_subprocess(gles_builders.build_gles2_headers),
+                "OpenGL_GLSL": env.Builder(
+                    action=run_in_subprocess(opengl_builders.build_opengl_headers),
                     suffix="glsl.gen.h",
                     src_suffix=".glsl",
                 )