PSP: zero-copy WindowSurface API for direct VRAM access

if all one needs is a raw framebuffer to the PSP's vram,
instead of dealing with renderers and textures, that need to be
copied hence and forth, this method allows one to create a window,
set the pixel format using SDL_SetWindowDisplayMode() - preferably
BGR565 for optimal speed (the other possible natively supported
option is ABGR8888) - and then request SDL_GetWindowSurface(),
which provides one with a surface with direct framebuffer access.
note that the pixels pointer inside the surface will be switched
after each call because of double-buffering.

it's advisable to overwrite all pixels of the PSP visible area
(480x272) to not encounter old data.

after writing the pixels, a call to SDL_UpdateWindowSurface()
sends the changes to the graphics chip.

the result is a raw framerate of 250 fps with BGR565 mode, under
optimal circumstances - i.e. nothing else is done than drawing,
and the drawing loop is as simple as possible.
that leaves about 12 ms per frame for other tasks and still allow
a fluent 60 fps.

src/render/psp/SDL_render_psp.c

@@ -36,6 +36,7 @@
 #include <stdarg.h>
 #include <stdlib.h>
 #include <vram.h>
+#include "SDL_render_psp.h"
 
 /* PSP renderer implementation, based on the PGE  */
 
@@ -124,6 +125,24 @@ typedef struct
     float x, y, z;
 } VertTCV;
 
+int SDL_PSP_RenderGetProp(SDL_Renderer *r, enum SDL_PSP_RenderProps which, void** out)
+{
+    PSP_RenderData *rd;
+    if (r == NULL) {
+        return -1;
+    }
+    rd = r->driverdata;
+    switch (which) {
+        case SDL_PSP_RENDERPROPS_FRONTBUFFER:
+            *out = rd->frontbuffer;
+            return 0;
+        case SDL_PSP_RENDERPROPS_BACKBUFFER:
+            *out = rd->backbuffer;
+            return 0;
+    }
+    return -1;
+}
+
 #define PI 3.14159265358979f
 
 #define radToDeg(x) ((x)*180.f / PI)

src/render/psp/SDL_render_psp.h

@@ -0,0 +1,32 @@
+/*
+  Simple DirectMedia Layer
+  Copyright (C) 1997-2024 Sam Lantinga <slouken@libsdl.org>
+
+  This software is provided 'as-is', without any express or implied
+  warranty.  In no event will the authors be held liable for any damages
+  arising from the use of this software.
+
+  Permission is granted to anyone to use this software for any purpose,
+  including commercial applications, and to alter it and redistribute it
+  freely, subject to the following restrictions:
+
+  1. The origin of this software must not be misrepresented; you must not
+     claim that you wrote the original software. If you use this software
+     in a product, an acknowledgment in the product documentation would be
+     appreciated but is not required.
+  2. Altered source versions must be plainly marked as such, and must not be
+     misrepresented as being the original software.
+  3. This notice may not be removed or altered from any source distribution.
+*/
+
+/* this header is meant to be included after the other related internal SDL
+   headers. it's the interface between psp renderer and video driver code. */
+
+enum SDL_PSP_RenderProps
+{
+    SDL_PSP_RENDERPROPS_FRONTBUFFER,
+    SDL_PSP_RENDERPROPS_BACKBUFFER,
+};
+
+int SDL_PSP_RenderGetProp(SDL_Renderer *r, enum SDL_PSP_RenderProps which, void** out);
+