This was probably missed when implementing luminance and luminance alpha
render targets.
_mesa_get_format_bits checks for both GL_*_BITS and GL_TEXTURE_*_SIZE.
This fixes:
main/framebuffer.c:892: _mesa_source_buffer_exists: Assertion `....' failed.
}
ASSERT(_mesa_get_format_bits(ctx->ReadBuffer->_ColorReadBuffer->Format, GL_RED_BITS) > 0 ||
_mesa_get_format_bits(ctx->ReadBuffer->_ColorReadBuffer->Format, GL_ALPHA_BITS) > 0 ||
+ _mesa_get_format_bits(ctx->ReadBuffer->_ColorReadBuffer->Format, GL_TEXTURE_LUMINANCE_SIZE) > 0 ||
+ _mesa_get_format_bits(ctx->ReadBuffer->_ColorReadBuffer->Format, GL_TEXTURE_INTENSITY_SIZE) > 0 ||
_mesa_get_format_bits(ctx->ReadBuffer->_ColorReadBuffer->Format, GL_INDEX_BITS) > 0);
break;
case GL_DEPTH: