Some extension may need to retrieve the `gdImagePtr` from an `GdImage`
object; thus, we export the respective function. To not being forced
to include gd.h in php_gd.h, we use the opaque `struct gdImageStruct *`
as return type.
We also rename php_gd2.dll to php_gd.dll, since there's not really much
point in giving the DLL a version number, since there is no php_gd.dll
for years (if there ever has been). Renaming, on the other hand,
matches the name on other systems (gd.so), and allows to actually use
`ADD_EXTENSION_DEP()`.
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