mesa: Fix error returned by glCopyTexImage2D() upon an invalid internal format

mesa: Fix error returned by glCopyTexImage2D() upon an invalid internal format

Page 161 of the OpenGL-ES 3.1 (PDF) spec, and page 207 of the OpenGL 4.5 (PDF),
both on section '8.6. ALTERNATE TEXTURE IMAGE SPECIFICATION COMMANDS', states:

    "An INVALID_ENUM error is generated if an invalid value is specified for
     internalformat".

It is currently returning INVALID_OPERATION error because
_mesa_get_read_renderbuffer_for_format() is called before the internalformat
argument has been validated. To fix this, we move this call down the validation
process, after _mesa_base_tex_format() has been called. _mesa_base_tex_format()
effectively serves as a validator for the internal format.

Fixes 1 dEQP test:
* dEQP-GLES3.functional.negative_api.texture.copyteximage2d_invalid_format

Fixes 1 piglit test:
* spec@oes_compressed_etc1_rgb8_texture@basic

Reviewed-by: Ian Romanick <ian.d.romanick@intel.com>
Cc: 10.6 <mesa-stable@lists.freedesktop.org>