mesa: Add ES31 API tag for the extension table.

I'll mark the OES_shader_image_atomic extension entry with this tag to
make sure that we don't expose it on earlier GLES API versions
accidentally, because according to the extension:

 "OpenGL ES 3.1 and GLSL ES 3.10 are required."

Reviewed-by: Tapani Pälli <tapani.palli@intel.com>

src/mesa/main/extensions.c

@@ -50,6 +50,7 @@ enum {
    ES1 = 1 << API_OPENGLES,
    ES2 = 1 << API_OPENGLES2,
    ES3 = 1 << (API_OPENGL_LAST + 1),
+   ES31 = 1 << (API_OPENGL_LAST + 2),
 };
 
 /**
@@ -773,6 +774,8 @@ _mesa_make_extension_string(struct gl_context *ctx)
    unsigned api_set = (1 << ctx->API);
    if (_mesa_is_gles3(ctx))
       api_set |= ES3;
+   if (_mesa_is_gles31(ctx))
+      api_set |= ES31;
 
    /* Check if the MESA_EXTENSION_MAX_YEAR env var is set */
    {
@@ -854,6 +857,8 @@ _mesa_get_extension_count(struct gl_context *ctx)
    unsigned api_set = (1 << ctx->API);
    if (_mesa_is_gles3(ctx))
       api_set |= ES3;
+   if (_mesa_is_gles31(ctx))
+      api_set |= ES31;
 
    /* only count once */
    if (ctx->Extensions.Count != 0)
@@ -880,6 +885,8 @@ _mesa_get_enabled_extension(struct gl_context *ctx, GLuint index)
    unsigned api_set = (1 << ctx->API);
    if (_mesa_is_gles3(ctx))
       api_set |= ES3;
+   if (_mesa_is_gles31(ctx))
+      api_set |= ES31;
 
    base = (GLboolean*) &ctx->Extensions;
    n = 0;