graphics: rewrite screenshot and api capture image processing (#870)

## Link to GitHub Issue or related Pull Request, if one exists
#0

## Description of change
Significantly speeds up API screen capture and D3D9 screenshots saving.
Two reasons for doing this:

1. We now have a 4K game (GITADORA) and existing capture code was taking
multiple seconds.
2. Renewed user interest on streaming as we have a couple more companion
apps in active development.

**API screen capture (streaming), 1280x720:** 14.3ms -> 6.3ms per frame.
Back buffer copies go to pooled `D3DPOOL_SYSTEMMEM` surfaces via
`GetRenderTargetData` instead of allocating a lockable render target
every frame, and TooJpeg is replaced with libjpeg-turbo (encode 9.8ms ->
3.0ms). MSAA remains unsupported

**Screenshots for GITADORA arena model, across 4 screens with one of
them 4K**: 4068ms -> 124ms. `D3DXSaveSurfaceToFileA` is replaced with
fpng (encode 4043ms -> 76ms) and the screens encode in parallel.
Dropping D3DX also removes the `d3dx9_43.dll` ... `d3dx9_24.dll` probing
loop, so screenshots no longer fail outright on machines with no D3DX9
runtime installed.

Screenshot surfaces are read on the present thread, so no D3D call
reaches another thread for screenshots. This fixes a hang in DDR X2
introduced earlier in the branch: its device has no internal locking,
and reading the surface on a pool thread while the present thread sat
inside `GetRenderTargetData` left the game's own render thread
deadlocked.


## Testing

- **GITADORA** (arena model, D3D9Ex, 4K main plus three subscreens,
windowed) with
`-screenshotsub`: three sets of four screenshots, images verified
correct. Completion
order differs between sets, so the screens really are encoding in
parallel.
- **LovePlus** (KLP, plain D3D9, 768x1360): covers the inline path used
by games whose
  image processing must not leave the present thread. 
- **API screen capture** through a companion app: live video correct
throughout.
- **Print Screen** bound as the screenshot key: the clipboard copy
succeeded on every shot.
- Quitting the game after capturing leaves no `IDirect3DDevice9`
reference count warning,
  so the pooled readback surfaces are released along with the device.

src/spice2x/external/libjpeg-turbo/jpegapicomp.h

@@ -0,0 +1,32 @@
+/*
+ * jpegapicomp.h
+ *
+ * Copyright (C) 2010, 2020, D. R. Commander.
+ * For conditions of distribution and use, see the accompanying README.ijg
+ * file.
+ *
+ * JPEG compatibility macros
+ * These declarations are considered internal to the JPEG library; most
+ * applications using the library shouldn't need to include this file.
+ */
+
+#if JPEG_LIB_VERSION >= 70
+#define _DCT_scaled_size  DCT_h_scaled_size
+#define _DCT_h_scaled_size  DCT_h_scaled_size
+#define _DCT_v_scaled_size  DCT_v_scaled_size
+#define _min_DCT_scaled_size  min_DCT_h_scaled_size
+#define _min_DCT_h_scaled_size  min_DCT_h_scaled_size
+#define _min_DCT_v_scaled_size  min_DCT_v_scaled_size
+#define _jpeg_width  jpeg_width
+#define _jpeg_height  jpeg_height
+#define JERR_ARITH_NOTIMPL  JERR_NOT_COMPILED
+#else
+#define _DCT_scaled_size  DCT_scaled_size
+#define _DCT_h_scaled_size  DCT_scaled_size
+#define _DCT_v_scaled_size  DCT_scaled_size
+#define _min_DCT_scaled_size  min_DCT_scaled_size
+#define _min_DCT_h_scaled_size  min_DCT_scaled_size
+#define _min_DCT_v_scaled_size  min_DCT_scaled_size
+#define _jpeg_width  image_width
+#define _jpeg_height  image_height
+#endif