main: Allow for the possibility of GL 3.2 without ARB_geometry_shader4.

Previously, we assumed that the only way Mesa would expose geometry
shader support was via the ARB_geometry_shader4 extension.  But this
extension has some extra complications over GL 3.2 (interactions with
compatibility-only features, and link-time initialization of the
constant gl_VerticesIn).  So we want to allow for the possibility of
supporting GL 3.2 (with GLSL 1.50 style geometry shaders) even if
ctx->Extensions.ARB_geometry_shader4 is false.

This patch adds a new function, _mesa_has_geometry_shaders(), which
returns true if either ARB_geometry_shader4 is supported or the GL
version is at least 3.2 desktop.  Since compute_version() only enables
GL 3.2 functionality when GLSL 1.50 support is present, a sufficient
way for a back-end to advertise geometry shader support is to set
ctx->Const.GLSLVersion >= 150.

v2: Remove unnecessary ctx->Const.GeometryShaders150 constant.

Reviewed-by: Ian Romanick <[email protected]>
Reviewed-by: Kenneth Graunke <[email protected]>

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