fix(renderer): disable window framebuffer alpha

[ClaudioBo]

Description

Disables alpha in the window framebuffer to prevent the compositor from interpreting it as window
transparency.

Tested on

• OS: CachyOS 7.0.12-1
• Desktop: KDE Plasma 6.27
• Display server: Wayland

Before

unfixed.mp4

After

fixed.mp4

[aalhendi]

We currently use framebuffer alpha as PS1 GPU metadata. The shader writes PS1 mask/STP state into the output alpha channel, and framebuffer readback packs that alpha back into PS1 VRAM bit 15.
Setting SDL_GL_ALPHA_SIZE = 0 can remove that storage from the default framebuffer, which risks breaking mask/STP behavior and framebuffer-feedback effects.

PSX Layout:
rrrrr ggggg bbbbb bit15

// Native:
  // when storing:
fragColor.a = ... ? 1.0 : 0.0; // storing bit15 into GL alpha channel
  // when renderer copies fb back into emulated psx vram
u8 a = (c >> 31) & 1;
*data_dst++ = r | (g << 5) | (b << 10) | (a << 15); // NOTE(aalhendi): alpha gets packed in bit 15

The safer fix is to separate emulation state from presentation.
We would probably keep PS1 mask/STP alpha in an internal RGBA target, then present an explicitly opaque final image to the SDL window.

Since this issue is compositor dependent, it is a low-priority target.