There is an specific list of texture targets that can be used with
glGetTextureImage. From OpenGL 4.5 spec, section '8.11 Texture Queries',
page 234 of the PDF:

   "An INVALID_ENUM error is generated if the effective target is
    not one of TEXTURE_1D , TEXTURE_2D , TEXTURE_3D , TEXTURE_1D_-
    ARRAY , TEXTURE_2D_ARRAY , TEXTURE_CUBE_MAP_ARRAY , TEXTURE_-
    RECTANGLE , one of the targets from table 8.19 (for GetTexImage
    and GetnTexImage only), or TEXTURE_CUBE_MAP (for GetTextureImage
    only)."

We are currently not validating the target for glGetTextureImage. As
an example, calling this function on a texture with target
GL_TEXTURE_2D_MULTISAMPLE should return INVALID_ENUM, but instead it
hits an assertion down the road in the i965 driver.

Reviewed-by: Nicolai Hähnle <nicolai.haehnle@amd.com>