Some compositors will send pointer enter/leave event while moving between surfaces that are part of the same window while mouse capture is active. Maintain window focus in this case, and adjust the coordinates relative to the content surface by the subsurface offset, if necessary.
XInput2 can send slave button presses before FocusIn events, which can confuse the click-through suppression logic. A window must have keyboard focus to grab the mouse anyway, so ignore slave presses when lacking keyboard focus.
This simply toggles a flag that rejects DnD offers if false. Events were previously dropped silently, but rejecting the offer makes some desktops display a proper icon when the drop will not work.
This fixes a UBSAN warning later in this function where it calculates
(1 << SDL_BITSPERPIXEL(surface->format)). The bpp might be >= 32 and
out of range for a bit shift.
SVE/SVE2 is a new SIMD extension for AArch64. Compared to NEON, SVE/SVE2 brings the following benefits that are good for SDL projects:
- Lane prediction: we don't have to treat the tail part of a stride separately when the width is n times the hardware vector size
- Although the performance is almost no difference from NEON when the hardware vector size is 128bits, when the hardware provides a longer vector size, e.g. 256, 512, ... 2048, we can enjoy the large performance gain without modifying the source code or recompiling a library.
The functional correctness is validated in a dedicated [qemu project](https://github.com/GorgonMeducer/aarch64_qemu_mac_template/tree/SDL-SVE2-Acceleration-Validation).
The performance is tested on [Radxa Orion 6 N](https://radxa.com/products/orion/o6n/), which provides 4x A720 and 4x A520 processors. Since the vector size is 128 bits, which is the same as NEON, the performance is almost the same (or no worse than) the NEON acceleration.
Because the blend mode is explicitly set to SDL_BLENDMODE_NONE, it doesn't matter if there's a transparency channel in the texture format or not for opaque windows. This ensures that a 32-bit format is used with Metal instead of SDL_PIXELFORMAT_RGB565.
Some compositors may dispatch this too early, during the initial empty commit, when subsurfaces are attached to a toplevel window, but a buffer has yet to be committed to the parent surface. Don't set the frame callback until the initial empty commit is done, so it will be called when the actual parent surface frame is committed.
