gl: Various issues with generated gir file

The meson build includes lots of EGLImage related types that the autotools build does not include, is this intentional? If it is

• gst_egl_get_error_string()'s return value has to be fixed so it is a type that gir actually knows about. EGLint it is not. Something like the following

@@ -8634,7 +8634,7 @@ you are writing to OpenGL. Conversely, combining #GST_MAP_GL with
       <parameters>
         <parameter name="err" transfer-ownership="none">
           <doc xml:space="preserve">an EGL error code</doc>
-          <type c:type="EGLint"/>
+          <type name="gint" c:type="EGLint"/>
         </parameter>
       </parameters>
     </function>

• GstGLMemoryEGLAllocator references GLMemoryEGLAllocatorClass as class struct, but the class struct is not actually in the gir file

CC @ystreet @nirbheek

Marking as blocker because this would cause regressions once meson is the only remaining build system :)