glapi/gen: Add code generation script for _mesa_create_exec_table().

This script generates the file api_exec.c, which contains just the
function _mesa_create_exec_table(), based on the XML files in
src/mapi/glapi/gen.

The following XML attributes, in particular, are used:
- "es1" indicates functions that should be available in ES1 contexts.
- "es2" indicates functions that should be available in ES2/ES3
  contexts.
- "exec" indicates which Mesa function should be dispatched to.  E.g.
  if the GL function is glFoo(), then:
  - exec="mesa" (the default) dispatches to _mesa_Foo().
  - exec="check" dispatches to _check_Foo().
  - exec="es" dispatches to _es_Foo().
  - exec="loopback" dispatches to loopback_Foo().
  - exec="skip" or exec="dynamic" causes this function to be skipped;
    either it is not yet supported ("skip"), or its dispatch table
    entry will be dynamically populated based on GL state ("dynamic").
- "desktop" indicates functions that should be available in desktop GL
  (non-ES) contexts.
- "deprecated" indicates functions that should not be available in
  core contexts.
- "mesa_name" indicates functions whose implementation in Mesa has a
  different suffix than the corresponding GL function name.

The generated code looks roughly like this (showing just a single
statement in each block for brevity):

    struct _glapi_table *
    _mesa_create_exec_table(struct gl_context *ctx)
    {
       struct _glapi_table *exec;

       exec = _mesa_alloc_dispatch_table(_gloffset_COUNT);
       if (exec == NULL)
          return NULL;

       if (_mesa_is_desktop_gl(ctx)) {
          SET_ActiveProgramEXT(exec, _mesa_ActiveProgramEXT);
          /* other functions not shown */
       }
       if (_mesa_is_desktop_gl(ctx) || _mesa_is_gles3(ctx)) {
          SET_BeginQueryARB(exec, _mesa_BeginQueryARB);
          /* other functions not shown */
       }
       if (_mesa_is_desktop_gl(ctx) || ctx->API == API_OPENGLES) {
          SET_GetPointerv(exec, _mesa_GetPointerv);
          /* other functions not shown */
       }
       if (_mesa_is_desktop_gl(ctx) || ctx->API == API_OPENGLES || ctx->API == API_OPENGLES2) {
          SET_ActiveTextureARB(exec, _mesa_ActiveTextureARB);
          /* other functions not shown */
       }
       if (_mesa_is_desktop_gl(ctx) || ctx->API == API_OPENGLES2) {
          SET_AttachShader(exec, _mesa_AttachShader);
          /* other functions not shown */
       }
       if (ctx->API == API_OPENGL) {
          SET_Accum(exec, _mesa_Accum);
          /* other functions not shown */
       }
       if (ctx->API == API_OPENGL || ctx->API == API_OPENGLES) {
          SET_AlphaFunc(exec, _mesa_AlphaFunc);
          /* other functions not shown */
       }
       if (ctx->API == API_OPENGLES) {
          SET_AlphaFuncxOES(exec, _es_AlphaFuncx);
          /* other functions not shown */
       }

       return exec;
    }
dispatch: Code generate api_exec.c.

This patch adjusts makefiles to cause src/mesa/main/api_exec.c to be
generated using src/mapi/glapi/gen/gl_genexec.py.  There should be no
functional change.
dispatch: Delete unused init_dispatch functions.

The new code-generated version of _mesa_create_exec_table() populates
the entire dispatch table (except for dynamic functions) by itself; it
no longer calls separate functions to initialize parts of the dispatch
table.  This patch removes those no-longer-needed functions.