render/gles2: don't constrain shm formats to ones that support reading

commit 44e8451c ("render/gles2: hide shm formats without GL support") added the is_gles2_pixel_format_supported() function to render/gles2/pixel_format.c, whose stated purpose is to "check whether the renderer has the needed GL extensions to read a given pixel format." It then used that function to filter the pixel formats returned by get_gles2_shm_formats().

The result of this change is that RGB formats are no longer reported for GL drivers that don't implement EXT_read_format_bgra, even when those formats are supported for rendering (which they have to be for wlr_gles2_renderer_create() to succeed). This is a pretty clear regression, since wlr_renderer_init_wl_shm() fails when either of WL_SHM_FORMAT_ARGB8888 or WL_SHM_FORMAT_XRGB8888 are missing.

To fix the regression, change is_gles2_pixel_format_supported() to accept all pixel formats that support rendering, regardless of whether we can read them or not, and move the check for EXT_read_format_bgra back into gles2_read_pixels(). (There's already a check for this extension in gles2_preferred_read_format(), so we're not breaking any abstraction that wasn't already broken.)

Tested on the NVIDIA 495.46 proprietary driver, which doesn't support EXT_read_format_bgra.

cc @mstoeckl