gles2n64: Substitute variables to clarify subsequent changesets.
gles2n64: Remove and simplify some unneeded code, clarify comments.

Currently, we never write display properties to the gles2n64 config
file, so the default values are always used:
   config.window.xpos = 0
   config.window.ypos = 0

So we simplify the code accordingly to preserve this behavior.

Also the width and height calculation was simplified to an equivalent
form that is more clear.
gles2n64: Save per-rom window config in separate variable.

Right now, the per-rom window sizing config is based on an assumption
that the user-provided size is always 800x480.  In the future, we want
to pass the actual display resolution via the config file.  So to
preserve the current behavior, we store the per-rom parameters as a
separate field in the structure.
gles2n64: Pass display resolution via config file; SDL2 compliant.

Passing the resolution this way avoids the need to call an SDL function
that has been removed in SDL2.