softpipe: set a lower minimum clamp value for texture coordinate border clamp

The value of -0.5f is not small enough to produce negative coordinates,
so lower the minimum clamp value to -1.0f. This fixes a number of tests
from
   dEQP-GLES31.functional.texture.border_clamp.*

Signed-off-by: Gert Wollny <gert.wollny@collabora.com>
Reviewed-by: Roland Scheidegger <sroland@vmware.com>

src/gallium/drivers/softpipe/sp_tex_sample.c

@@ -318,7 +318,7 @@ static void
 wrap_linear_clamp_to_border(float s, unsigned size, int offset,
                             int *icoord0, int *icoord1, float *w)
 {
-   const float min = -0.5F;
+   const float min = -1.0F;
    const float max = (float)size + 0.5F;
    const float u = CLAMP(s * size + offset, min, max) - 0.5f;
    *icoord0 = util_ifloor(u);