Revert "mesa: simplify _mesa_is_image_unit_valid for buffers"

This reverts commit c0ed52f614.  It was
preventing the image format validation from being done on buffer
textures, which is required to ensure that the application doesn't
attempt to bind a buffer texture with an internal format incompatible
with the image unit format (e.g. of different texel size), which is
not allowed by the spec (it's not allowed for *any* texture target,
whether or not there is spec wording restricting this behavior
specifically for buffer textures) and will cause the driver to
calculate texel bounds incorrectly and potentially crash instead of
the expected behavior.

Cc: mesa-stable@lists.freedesktop.org
Reviewed-by: Marek Olšák <marek.olsak@amd.com>
Bugzilla: https://bugs.freedesktop.org/show_bug.cgi?id=106465
Reviewed-by: Nanley Chery <nanley.g.chery@intel.com>

src/mesa/main/shaderimage.c

@@ -478,13 +478,6 @@ _mesa_is_image_unit_valid(struct gl_context *ctx, struct gl_image_unit *u)
    if (!t)
       return GL_FALSE;
 
-   /* The GL 4.5 Core spec doesn't say anything about buffers. In practice,
-    * the image buffer format is always compatible with the underlying
-    * buffer storage.
-    */
-   if (t->Target == GL_TEXTURE_BUFFER)
-      return GL_TRUE;
-
    if (!t->_BaseComplete && !t->_MipmapComplete)
        _mesa_test_texobj_completeness(ctx, t);
 
@@ -498,14 +491,20 @@ _mesa_is_image_unit_valid(struct gl_context *ctx, struct gl_image_unit *u)
        u->_Layer >= _mesa_get_texture_layers(t, u->Level))
       return GL_FALSE;
 
-   struct gl_texture_image *img = (t->Target == GL_TEXTURE_CUBE_MAP ?
-                                   t->Image[u->_Layer][u->Level] :
-                                   t->Image[0][u->Level]);
+   if (t->Target == GL_TEXTURE_BUFFER) {
+      tex_format = _mesa_get_shader_image_format(t->BufferObjectFormat);
 
-   if (!img || img->Border || img->NumSamples > ctx->Const.MaxImageSamples)
-      return GL_FALSE;
+   } else {
+      struct gl_texture_image *img = (t->Target == GL_TEXTURE_CUBE_MAP ?
+                                      t->Image[u->_Layer][u->Level] :
+                                      t->Image[0][u->Level]);
+
+      if (!img || img->Border || img->NumSamples > ctx->Const.MaxImageSamples)
+         return GL_FALSE;
+
+      tex_format = _mesa_get_shader_image_format(img->InternalFormat);
+   }
 
-   tex_format = _mesa_get_shader_image_format(img->InternalFormat);
    if (!tex_format)
       return GL_FALSE;