graphics: rewrite image scaler (#278)

## Link to GitHub Issue, if one exists
Fixes #276 

## Description of change

Note: this change is not compatible with previous screen resize config;
i.e., if you had the zoom setting dialed into a certain view, after this
change it won't be the same area, so you'll have to set up a scene
again. I think this is a worthwhile cost since the old logic is just
broken in weird ways, and preserving old settings is really tedious math
(for a feature that not many people use). Besides, this change will only
use the new Scenes values from the JSON.

Rewrite the DX9 image scaler logic.

* Previously, the rendering surface was fixed at `4096*4096 px`. Now,
this is relative to the backbuffer dimensions, which depends on the game
& respects user provided `-forceres`.
* Remove "center" option, make it the default.

Above two changes fix the aspect ratio issue (scaling in portrait games
not respecting screen ratio) and things breaking at higher res (1080p
when zoomed out far enough, or when forced to run at 4k resolution for
example)

* Use `scenes` leaf of screen resize JSON config for Scene 1 as well,
completely removing ourselves from being compatible with older spice2x
versions for these values.
* Add additional bounds check before calling `StretchRect` to avoid
users getting stuck with a bad layout, which can kill performance.

## Compiling
🦾 

## Testing
Tested 32 bit (popn / ddr), 64 bit (ldj), portrait and landscape modes
(kfc)

src/spice2x/overlay/windows/sdvx_sub.cpp

@@ -35,7 +35,7 @@ namespace overlay::windows {
             default:
                 this->init_size = ImVec2(ImGui::GetIO().DisplaySize.x - (padding * 2), 0.f);
                 this->init_size.y = (this->init_size.x * 9 / 16) + ImGui::GetFrameHeight();
-                if (GRAPHICS_FS_FORCE_LANDSCAPE) {
+                if (GRAPHICS_FS_ORIENTATION_SWAP) {
                     this->init_size.x /= 2;
                     this->init_size.y /= 2;
                 }