mesa: fix tex format selection for GL_R32F and other R/G float formats

mesa: fix tex format selection for GL_R32F and other R/G float formats

The i965 driver advertises GL_ARB_texture_float and GL_ARB_texture_rg
support but the ctx->TextureFormatSupported[] table entries for
MESA_FORMAT_R_FLOAT32 and MESA_FORMAT_RGBA_FLOAT32 are false on gen 4
hardware.  So the case for GL_R32F would fail and we'd print an
implementation error.

This patch adds more Mesa tex format options for GL_R32F and other R/G
formats so we fall back to 16-bit formats when 32-bit formats aren't
available.

Eric made the same fix in commit 6216a5b4 for the non R/G formats.

v2: try 16-bit formats before 32-bit formats and try RG formats before
RGBA where possible.

This should fix https://bugs.freedesktop.org/show_bug.cgi?id=44039

NOTE: This is a candidate for the 8.0 and 7.11 branches.

Reviewed-by: José Fonseca <jfonseca@vmware.com>
Reviewed-by: Roland Scheidegger <sroland@vmware.com>