We'll use this function from wlr_shm too.
Add some assertions, use int32_t (since the wire protocol uses that,
and we don't want to use 16-bit integers on exotic systems) and
switch the stride check to be overflow-safe.
These formats require EXT_texture_norm16, which in turn needs OpenGL
ES 3.1. The EXT_texture_norm16 extension does not support passing
gl_internalformat = GL_RGBA to glTexImage2D, as can be done for
formats available in OpenGL ES 2.0, so this commit adds a field to
wlr_gles2_pixel_format to provide a more specific internalformat
parameter to glTexImage2D.
