gallium: add PIPE_SHADER_CAP_MAX_OUTPUTS and use it in st/mesa

With 5 shader stages and various combinations of enabled and disabled shaders,
the maximum number of outputs in one shader doesn't have to be equal to
the maximum number of inputs in the following shader.

v2: return 32 for softpipe and llvmpipe

src/gallium/docs/source/screen.rst

@@ -267,6 +267,8 @@ support different features.
 * ``PIPE_SHADER_CAP_MAX_TEX_INDIRECTIONS``: The maximum number of texture indirections.
 * ``PIPE_SHADER_CAP_MAX_CONTROL_FLOW_DEPTH``: The maximum nested control flow depth.
 * ``PIPE_SHADER_CAP_MAX_INPUTS``: The maximum number of input registers.
+* ``PIPE_SHADER_CAP_MAX_OUTPUTS``: The maximum number of output registers.
+  This is valid for all shaders except the fragment shader.
 * ``PIPE_SHADER_CAP_MAX_CONST_BUFFER_SIZE``: The maximum size per constant buffer in bytes.
 * ``PIPE_SHADER_CAP_MAX_CONST_BUFFERS``: Maximum number of constant buffers that can be bound
   to any shader stage using ``set_constant_buffer``. If 0 or 1, the pipe will