Instead of using the desktop <GL/gl.h>, we properly use the OpenGL ES 3.x
headers. Also, we now use GL_EXT_buffer_storage for MAP_PERSISTENT_BIT_EXT
and MAP_COHERENT_BIT_EXT as the core versions are only available in desktop
OpenGL 4.4. Similarly, we need GL_EXT_texture_format_BGRA8888 for GL_BGRA_EXT
as GL_BGRA is desktop-only.
We ask for 32-bit colour buffer when creating the EGL context. On Wayland,
this sometimes give contexts with alpha channels, resulting in unwanted
transparency. So we clear the alpha channel in the desktop shader.
We also switch to using constant alpha for blending the splash, which
avoids more alpha issues.
